Sciencemadness Discussion Board - Chemistry brain teasers! Noob Competition!

When is the closing date? I am going to have a crack at this, i like problems like this.

That will depend on participation levels and available time (mine).

Each problem will be given at least 4 days to allow max participation. In order not to drag things out and exceed average attention spans I anticipate 5 - 7 teasers in total (about 1 month competition duration).

NEMO-Chemistry - 28-8-2016 at 06:24

Great i was thinking in hours!!! Feel free to publish my answer, especially if i get it wrong!

My reasoning is more people might have a go if they see there is no shame in getting stuff wrong, having a go is what matters.

blogfast25 - 28-8-2016 at 15:36

OOOOPSIE:

BIG, Big 'my bad'.

Due to a typo on my part, the volume of CO2 eluted should have been stated as 952 ml at STP, NOT 476 ml. The original question has been edited to reflect that and is now CORRECT.

Several good answers were received but none correct, obviously due to that reason.

Due to that reason, please adjust your answers, if you can. But two members had it right and need not adjust their calcs, they have +10 in my book. Well done both (they have been U2Ued).

In the 'egg on face competition' I've now got an UNBEATABLE lead. :mad:

Please accept apologies.

[Edited on 28-8-2016 by blogfast25]

j_sum1 - 28-8-2016 at 18:03

Thanks for doing this blogfast.
I won't enter the competition since I am not that kind of noob. (I am a noob in plenty of other areas though.)
I did do the problem and enjoyed the puzzle element of it. Nice style of question.

I might keep track of progress on this thread and pinch ideas as they come up.
J.

blogfast25 - 28-8-2016 at 18:22

Quote: Originally posted by j_sum1

Thanks for doing this blogfast.
I won't enter the competition since I am not that kind of noob. (I am a noob in plenty of other areas though.)
I did do the problem and enjoyed the puzzle element of it. Nice style of question.

I might keep track of progress on this thread and pinch ideas as they come up.
J.

Thanks J_sum1.

Or formulate your own problem (as part of the competition, I could do with some help here)? No typos please!

[Edited on 29-8-2016 by blogfast25]

j_sum1 - 28-8-2016 at 19:01

Yeah. I can probably do that.

NEMO-Chemistry - 28-8-2016 at 23:20

Now my answer makes sense!! I wont say why because it will give it away.

I couldnt work out why it fitted in one area but didnt fit in the other area

. I will submit later once i had a chance to double check and type all my reasoning out.

Lets see if i can make it 3 correct

. If it's any consolation Blogfast the error made me read up on the ideal gas equation. So if nothing else i have ventured into previously unknown territory.

diddi - 28-8-2016 at 23:38

I might be able to help j_sum.

Sulaiman - 29-8-2016 at 01:04

I think a clear definition of STP is required

j_sum1 - 29-8-2016 at 01:18

I think a clear definition of STP is required

I thought that was reasonably well established.
https://en.wikipedia.org/wiki/Standard_conditions_for_temper...

Sulaiman - 29-8-2016 at 01:30

try using those conditions for this definition https://en.wikipedia.org/wiki/Molar_volume
then try to find the mystery alkali earth metal atomic weight

though the question did say "most likely" so close enough I suppose.
but then why the precision in starting weight and collected volume ?

Going backwards I get 22.3855 l/mole at 'STP' ?

Since i'm sure to get flamed I'm going to re-check .....

Edit: checked .. 22.3855 l/mole at 'blogfast25's STP'

[Edited on 29-8-2016 by Sulaiman]

Yes I'm being difficult ... so would you if last thing at night you have to invent a new super-heavy alkaline earth metal

[Edited on 29-8-2016 by Sulaiman]

PHILOU Zrealone - 29-8-2016 at 03:39

OOOOPSIE:

BIG, Big 'my bad'.

Due to a typo on my part, the volume of CO2 eluted should have been stated as 952 ml at STP, NOT 476 ml. The original question has been edited to reflect that and is now CORRECT.

Several good answers were received but none correct, obviously due to that reason.

Due to that reason, please adjust your answers, if you can. But two members had it right and need not adjust their calcs, they have +10 in my book. Well done both (they have been U2Ued).

In the 'egg on face competition' I've now got an UNBEATABLE lead. :mad:

Please accept apologies.

[Edited on 28-8-2016 by blogfast25]

You look more human when doing such mistakes

.

Don't blame yourself!
I have followed the cursus to become a teacher of chemical sciences for upper secondary school (pupils from 15 to 18 years old and high school or university).
One of the hardest/tough part of the work is to make the questions for the test...because you have to think backwards and this may lead to mistakes (example like here division instead of multiplication).

If you really want to avoid such mistakes...you'll have to take a minute to pass the test and check if you get the expected answer...then all mistakes into the introductive text will become apparent.

j_sum1 - 29-8-2016 at 04:12

try using those conditions for this definition https://en.wikipedia.org/wiki/Molar_volume
then try to find the mystery alkali earth metal atomic weight

though the question did say "most likely" so close enough I suppose.
but then why the precision in starting weight and collected volume ?

Going backwards I get 22.3855 l/mole at 'STP' ?

Since i'm sure to get flamed I'm going to re-check .....

Edit: checked .. 22.3855 l/mole at 'blogfast25's STP'

[Edited on 29-8-2016 by Sulaiman]

Yes I'm being difficult ... so would you if last thing at night you have to invent a new super-heavy alkaline earth metal

[Edited on 29-8-2016 by Sulaiman]

I suspect you have missed something as you have attempted the calculation. I am not going to try to unravel it. To do so would likely spill the answer and I don't want to do that.
Since you have already calculated it however, it is a common shorthand that "one mole of any gas at STP occupies 22.4 litres". No in-depth knowledge of gas laws required.

The flip side of this is, if you do discover a superheavy group 2 element, that is pretty exciting. Element 120 has not been reported yet and Sulaimanium has a nice sound to it.

NEMO-Chemistry - 29-8-2016 at 04:33

Be careful guys please, i have the answer to both questions. BUT the reasoning needs me to add in the gas and this is something i am working on.

Please dont give clues i really need to learn this. I took STP to mean 0C (But in Kelvin) at 1Atmosphere of pressure, my trouble is i am slightly out on another calculation.

I know this is a rounding error as its so close in every other way and nothing else is remotely as close. This is how i worked it out, but i want/need to link my reasoning to the gas volume.

I know the question dosnt exactly tell you that the reasoning needs to include the gas, but clearly that is the intended implication of putting the figure there.

I got my answer last night (I am pretty sure i am right) but realized i dont know how to work out the gas volume, so this i am doing now.

DJF90 - 29-8-2016 at 05:05

Going backwards I get 22.3855 l/mole at 'STP' ?

Since i'm sure to get flamed I'm going to re-check .....

Edit: checked .. 22.3855 l/mole at 'blogfast25's STP'

I suspect the small difference is due to the intended meaning of STP. I've always taken it to mean 298 K and 1 Atm, but that wikipedia link suggests it refers to 278 K (0 *C). I think blogfast needs to clarify for the benefit of all, but suspect his interpretation is the same as mine.

blogfast25 - 29-8-2016 at 05:10

@Philou: haste makes waste, I didn't try and solve it myself, even though I 'always' do.

@Suleiman: in the corrected version the answer is now unequivocal. Sorry for any inconvenience caused.

Some more answers have come in.

Solution provided tomorrow. All: thanks for playing!

Sulaiman - 29-8-2016 at 06:21

I like it ...
after giving such a simplified assumption = wrong for the first quiz I thought I'd be more careful ...

blogfast25 - 29-8-2016 at 06:52

I like it ...
after giving such a simplified assumption = wrong for the first quiz I thought I'd be more careful ...

Have you sent in an answer yet?

Note to non-noobs:

Several of you chose to answer and that's perfectly ok with me: the more the merrier. But I expect any non-noob winners to be charitable and relinquish their prize to the first noob runner up.

aga - 29-8-2016 at 07:01

What a great Puzzle !

I've had a go, but seem to have tripped up somewhere ...

blogfast25 - 29-8-2016 at 07:52

What a great Puzzle !

I've had a go, but seem to have tripped up somewhere ...

A minor mistake made in good faith. Near full marks. Start training for the second lap to make up for the few lost seconds. And DON'T forget your tyre repair kit!

aga - 29-8-2016 at 08:16

DON'T forget your tyre repair kit!

Round 2 is a low-cost boat race on car tyre innertubes !

I knew it.

NEMO-Chemistry - 29-8-2016 at 11:20

Ok my entry is in, i am not as confident as i was!! Still its a great chance to learn and alot of fun.

NEMO-Chemistry - 29-8-2016 at 12:55

Well fuffed that up!! a darn good thrashing with the noob stick required!

DraconicAcid - 29-8-2016 at 12:57

*Thrash* *thrash* *snap*

Dammit. Now I need a new N00b stick.

NEMO-Chemistry - 29-8-2016 at 13:04

*Thrash* *thrash* *snap*

Dammit. Now I need a new N00b stick.

Better get a new one for the next question!!!

Lets hope i dont get to liking the beatings lol

aga - 29-8-2016 at 13:16

Erm, the SM stands for ScienceMadness, not Sado-Masochism ...

DraconicAcid - 29-8-2016 at 13:18

That's okay, I needed a new one before the start of classes anyway.

blogfast25 - 29-8-2016 at 15:07

Next question, Tue 29/8, will come from DraconicAcid, after my answer to #1.

Please answer to DraconicAcid, not your humble undersigned.

Keep answers concise and to the point, like exam answers, and not like:

Quote:

Dear Teacher,

I was thinking about it the other day and while peeling the cabbages I saw the light. So blah blah...

Thank you!

[Edited on 29-8-2016 by blogfast25]

aga - 29-8-2016 at 15:47

Wow ! That's freaky on quite a few levels.

Anyway, it's 30-8-2016 here, so dump the puzzle on us Draco.

Maybe that's the Puzzle ?

If the puzzle had-was-having-hadded Time-Shift involved, i'd better will-have-gotted-getting-get really drunk to even understand it.

blogfast25 - 29-8-2016 at 16:40

Answer to first question:

952 ml CO₂ at STP is:

$$n=\frac{0.952 L}{22.4 L/mole}=0.0425\:\mathrm{mole}$$

Half of that is MgCO₃, so:

$$\frac12 \times 0.0425 \times 84.134=1.79\:\mathrm{g}$$

... of MgCO₃. That leaves:

$$4.932-1.79=3.144\:\mathrm{g}$$

... of the other carbonate. We also know it's 0.02125 moles.

The molar mass of XCO₃ is thus:

$$MM=\frac{3.144}{0.02125}=148\:\mathrm{g/mole}$$

... which is very close to the MM of SrCO₃.
<hr>

DraconicAcid can now publish Question #2. Thanks.

ficolas - 29-8-2016 at 22:48

That wasnt 4 days

I finished the problem yesterday at 11-12 pm and decided to wait for tomorrow (today) so that I could check it in a decent hour, and I come here to find out in that time the answer was posted

I'll just be faster with the second one I guess

Edit: Now I see the "solution provided tomorrow", woops xD

[Edited on 30-8-2016 by ficolas]

NEMO-Chemistry - 29-8-2016 at 22:54

I was telling you about peeling Carrots, NOT Cabbages!

Well i was well out, but handy seeing the answer. So something learned

DraconicAcid - 29-8-2016 at 23:31

3.75 g of a metal chloride is dissolved in water and treated with excess aqueous silver nitrate, resulting in the formation of 10.66 g of precipitate.

a) (3 marks) Give the net ionic equation for the formation of the precipitate (with states and charges).

b) (7 marks) Identify the metal, giving your reasoning.

U2U me your answers (don't worry, I haven't found a new stick yet).

Lefaucheux10 - 30-8-2016 at 00:17

hum interresting but i found severals solutions. I think we need one more information to be certain of to have only one solution ! Can you give this information or not ?

I take the anh forme of a metal chloride for resolve the problem is that correct ?

CuReUS - 30-8-2016 at 00:32

That wasnt 4 days

I finished the problem yesterday at 11-12 pm and decided to wait for tomorrow (today) so that I could check it in a decent hour, and I come here to find out in that time the answer was posted

same here

, it was so easy

Quote: Originally posted by Lefaucheux10

I think we need one more information to be certain of to have only one solution !

yeah,you are right. the question is incomplete.

[Edited on 30-8-2016 by CuReUS]

DraconicAcid - 30-8-2016 at 00:37

It is not incomplete. Search your feelings, young padawan....you know it to be true.

Quote:

hum interresting but i found severals solutions.

And is there no flaw in any of those solutions?

[Edited on 30-8-2016 by DraconicAcid]

NEMO-Chemistry - 30-8-2016 at 00:43

Suck it up boys suck it up. I got caught on the last question because i approached the problem from the wrong angle, if there is several solutions then you can bet your missing something.

If they are too easy your not a true noob

. How long do we get for this one?

DraconicAcid - 30-8-2016 at 00:49

How long do we get for this one?

Let's say, 48 hours, starting......NOW!

Neme - 30-8-2016 at 01:57

I can confirm there is only one solution.

MeshPL - 30-8-2016 at 02:42

I'm not a noob, when it comes to theoretical (paper) chemistry (practice is a different thing), but I decided to give this compoetition a try. I missed the first one, but whatever. I've found a solution (pretty easily) but I'm not sure how to send it via U2U. The equation is a big problem. Is it possible to send a file (doc or pdf of even png) via U2U?

BTW, will there be a question about organic chemistry?

j_sum1 - 30-8-2016 at 03:03

Great question DA. I really enjoyed doing that one. Rather a clever concept.

@MeshPL.
There has been a bit of discussion behind the scenes with blogfast and questions coming from a few different sources. I don't know if there is an OC one in the pipeline but I hope so. If there is, it is likely to stretch me.

aga - 30-8-2016 at 04:22

Easy ?

A lot of head-scratching going on here.

Edit:

Finally worked it out !

There does seem to be only 1 right answer.

[Edited on 30-8-2016 by aga]

blogfast25 - 30-8-2016 at 06:01

Important notice:

"Sorry" to those missed a deadline.

After DraconicAcid's question is closed, several more questions will be published more or less at once and these will stay open till the end of the competition (back-end of September, exact date and time to be decided on yet).

NEMO-Chemistry - 31-8-2016 at 04:24

Important notice:

"Sorry" to those missed a deadline.

After DraconicAcid's question is closed, several more questions will be published more or less at once and these will stay open till the end of the competition (back-end of September, exact date and time to be decided on yet).

I am still head scratching! Doing the questions at once and then a reasonable deadline would be great, i can manage my time better then.

ficolas - 31-8-2016 at 07:52

How should I send the answer?
LatEx doesnt seem to work in u2u and I have a hard to understand handwritting, and a non latex formated answer may be hard to read.
Not everybody knows latex anyways (The reason I tried latEx is because of my shitty handwritting), I just learned some of it because my sister, who is very bad with computers needed to learn it to make stuff for his degree, so I decided to learn it to help her, and ill probably need it in the future once I start my chem. E. degree in like a month

Ill send this answer as an screenshot to a latEx thingie

[Edited on 31-8-2016 by ficolas]

NEMO-Chemistry - 31-8-2016 at 08:45

How should I send the answer?
LatEx doesnt seem to work in u2u and I have a hard to understand handwritting, and a non latex formated answer may be hard to read.
Not everybody knows latex anyways (The reason I tried latEx is because of my shitty handwritting), I just learned some of it because my sister, who is very bad with computers needed to learn it to make stuff for his degree, so I decided to learn it to help her, and ill probably need it in the future once I start my chem. E. degree in like a month

Ill send this answer as an screenshot to a latEx thingie

[Edited on 31-8-2016 by ficolas]

It says to send the answer to Draconic Acid by U2U.

P.S give me the answer and i will forwarded it for you

p.s please dont send me the answer! it was a joke (just in case...)

DraconicAcid - 31-8-2016 at 08:49

It's okay- ficolas has sent me his answer.

ficolas - 31-8-2016 at 08:51

Quote:

It says to send the answer to Draconic Acid by U2U.

P.S give me the answer and i will forwarded it for you

p.s please dont send me the answer! it was a joke (just in case...)

I meant like the format of the answer, a picture to a notebook, plain text, latex, etc

NEMO-Chemistry - 31-8-2016 at 09:05

It's okay- ficolas has sent me his answer.

Thanks for rubbing it in

, mine will be in by tonight (hopefully).

Lefaucheux10 - 31-8-2016 at 09:05

It's okay- ficolas has sent me his answer.

have you mine DraconicAcid ?

NEMO-Chemistry - 31-8-2016 at 09:07

Quote: Originally posted by Lefaucheux10

It's okay- ficolas has sent me his answer.

have you mine DraconicAcid ?

Go away!!!! I dont need telling everyone else has done it thank you!

dont worry i am not serious, well done for working it out.

DraconicAcid - 31-8-2016 at 09:18

I have replies from Neme, CeReUS, Lafrauchex10, aga, Sulaiman, MeshPL, PHILOU Zrealone, Zandins, nezza and ficolas. Nobody has gotten it so wrong that I've had to pull out the N00b stick.

ficolas - 31-8-2016 at 09:20

Sending it doesnt mean getting it right anyways :s

Im having quite a lot of fun with these questions, they dont require a lot of chemistry knowledge, but they require thinking. With the stuff that we have learned at high school, it is possible to do them, but if this was in a high school exam most of the class would fail it. Its the kind of question I like

I remember 3 or 4 years ago, my math teacher talked with me because in an exams, I went direct to the hard question (they always had 1 hard question), I spent too much time in it and I didnt finish the whole exam and I always did the hard question the first one xD
It was something stupid to do but I was youngh...er

aga - 31-8-2016 at 09:20

Guys. Careful. He's obviously got a new N00b stick.

Probably bigger and more durable than the last one, maybe even the N00c model.

[Edited on 31-8-2016 by aga]

blogfast25 - 31-8-2016 at 09:44

I have replies from Neme, CeReUS, Lafrauchex10, aga, Sulaiman, MeshPL, PHILOU Zrealone, Zandins, nezza and ficolas. Nobody has gotten it so wrong that I've had to pull out the N00b stick.

When are you closing, DA?

DraconicAcid - 31-8-2016 at 10:00

I have replies from Neme, CeReUS, Lafrauchex10, aga, Sulaiman, MeshPL, PHILOU Zrealone, Zandins, nezza and ficolas. Nobody has gotten it so wrong that I've had to pull out the N00b stick.

When are you closing, DA?

In about 13 hours.

aga - 31-8-2016 at 11:15

When are you closing, DA?

What ? We were supposed to answer in Russian ?

Oh crap.

NEMO-Chemistry - 31-8-2016 at 11:30

When are you closing, DA?

What ? We were supposed to answer in Russian ?

Oh crap.

Thats me in it then, i answer in double Dutch.

aga - 31-8-2016 at 11:57

Jeez. Just 11 hours to go and we had to learn Russian !

DraconicAcid is a hard task-master.

1. Мой ответ является правильным, даже если это не так, товарищ.

2. Дайте полную марок меня или я буду есть ваши хомяков - все они, один за другим

[Edited on 31-8-2016 by aga]

DraconicAcid - 31-8-2016 at 12:09

К счастью , у меня нет хомяков , так что ваши угрозы не влияют на меня .

Texium - 31-8-2016 at 12:13

Now we need Ave to show up and tell you guys that you're completely wrong because you probably just plugged stuff into Google Translate

aga - 31-8-2016 at 12:23

Run NEMO ! Run !

They're onto us already.

NEMO-Chemistry - 31-8-2016 at 13:10

Why should i run?? Wasnt me who just ordered a Sulphuric acid colonic irrigation in Russian

.

Just what you boys need.....a good ole clear out lol. I have had to skip this question, I am working on Molarity and associated stuff to catch up with the course at school. I found the question hard with my current level of knowledge.

So i have decided to not to rush this one, i would rather study it carefully and do the chemistry homework i have. I am not dropping out of the competition though.

For me its not about winning but about learning, i have done alot of reading for the question, so hopefully when i see the answer alot of things will click into place. Great question though, and thanks to aga for saying it made him head scratch!

After so many said it was easy i felt bad that i didnt find it so easy, so having someone who is fairly experienced say they had to work at it made me feel better.

This really is a great way to learn, with or without the prize incentive its alot of fun.

blogfast25 - 31-8-2016 at 13:45

Crrraaaaaack! (sound of long metal ruler hitting desk!)

Silence in the study room, please!

(It's always the same g-ddamn suspects, isn't it? One of 'em's only been back for half a day an'es already flirting with detention again!)

[Edited on 31-8-2016 by blogfast25]

NEMO-Chemistry - 31-8-2016 at 13:50

aga - 31-8-2016 at 13:51

I spent about 3 hours on it, so yes, quite hard for a noob, which i consider myself to be.

If you imagine the whole Molecular weight thing as just how many grammes one mol of some random stuff weighs, and how much 1 litre of solvent is, Molarity gets easier.

e.g. 1 mol of NaCl = 58.469 grammes.

Stuff 58.44 g of NaCl into 1 litre of water, and you got a 1 molar solution.

a.k.a. [1] M NaCl solution.

Then you can use C1V1= C2V2 to dilute that down to any molar concentration you want.

e.g. you need [0.5] M NaCl solution, and you got that 1 litre with 58.44g salt in it.

C1 = [1], V1=1
C2 = [0.5], V2 = ?

Do some light algebra and V2 = (C1 V1) / C2

So V2 = 1 x 1 / 0.5 = 2, so dilute it to 2 litres and it's Molarity reduces to [0.5]

Obvious going from 1 to 0.5, but the maths hold true for any desired concentration.

Oh my lord.
blogfast25 seems to have the metal N00d model stick.

Edit:

blogfast25 taught me that, hope i passed it on ok.

[Edited on 31-8-2016 by aga]

j_sum1 - 31-8-2016 at 14:44

Hey. I want nothing to do with blogfast's nood stick.

[blocks ears, closes eyes]
La la la la la la la la la la
[/bece]

blogfast25 - 1-9-2016 at 12:37

4 more problems will now be published. These will stay open till the end of the competition (end of this month, precise date to be published soon). More may yet be published shortly...

Please U2U all concise answers with summarised reasoning to ME. Use the red reference number (top left) as the title of your U2U.

Thank you for playing!

DraconicAcid - 1-9-2016 at 12:42

The deadline for my question has passed, so I guess I ought to post the solution.

3.75 g of a metal chloride is dissolved in water and treated with excess aqueous silver nitrate, resulting in the formation of 10.66 g of precipitate.

Net ionic equation: Ag⁺(aq) + Cl^-(aq) --> AgCl(s)

We obtained 10.66 g of silver chloride, which is 74.37 mmol. *If* the metal was monovalent, that would mean the molar mass of the metal chloride was 3.75 g / 0.07437 mol = 50.4 g/mol, which would give the metal a molar mass of 15.0 g/mol. This is not the molar mass of a metal.

If it's divalent, then the molar mass of the metal chloride would be 3.75 g / (0.07437 mol/ 2) = 100.8 g/mol. Subtracting the two chlorides gives 29.9 g/mol. This is between silicon and phosphorus, which are not metals.

Trivalent gives 44.9 g/mol, which is within experimental error for scandium (which is indeed trivalent).

Tetravalent gives 59.9 g/mol, which is close to cobalt or nickel, neither of which form tetrachlorides.

Pentavalent gives 74.9 g/mol, which would be arsenic, but arsenic is not a metal, and its pentachloride is not ionic.

Hexavalent would give 89.8 g/mol, which is close to yttrium and zirconium, neither of which have a +6 oxidation state. Similarly, rhodium and palladium do not form heptachlorides, and tin doesn't form an octachloride. So the metal must be scandium.

[Edited on 1-9-2016 by DraconicAcid]

blogfast25 - 1-9-2016 at 12:44

NC#3

(Courtesy of j_sum1)

A fertiliser is known to be a combination of ammonium sulfate, ammonium nitrate and potassium nitrate.

Distillation of 100g of the fertiliser with sulfuric acid yields 41.5g of white fuming nitric acid (100 % pure nitric acid).

Addition of excess sodium hydroxide to 100g of the fertilizer releases 15.6g of ammonia gas.

Dissolving 100g of the fertiliser in water and adding excess calcium nitrate solution gives a white precipitate. After filtering, drying and calcining to obtain the anhydrous form, 41.2g of this precipitate is recovered.

Question:

Assume that the original fertiliser is fully dry and that there is 100% conversion for each of the tests. What is the composition of the fertiliser?

3 points per component.

[Edited on 1-9-2016 by blogfast25]

aga - 1-9-2016 at 12:50

The deadline for my question has passed, so I guess I ought to post the solution.

Net ionic equation: Ag⁺(aq) + Cl^-(aq) --> AgCl(s)

... and Scandium (hopefully).

OK. Learning Hindi now for the next question.

Edit:

Wonderful puzzles guys ! Thanks for posting them !

[Edited on 1-9-2016 by aga]

blogfast25 - 1-9-2016 at 12:56

NC#4

5.000 g of a specific hydrocarbon (C_xH_y) is quantitatively oxidised to CO₂ and H₂O. The water is caught in a cold trap and 4.49 g of it obtained.

The CO₂ is quantitatively converted to BaCO₃ and 73.9 g of it obtained.

Questions:

1. What is the simplest empirical formula of the compound? (7 points)

2. Which of the following could it be? (3 points)

a. methyl benzene
b. butyl benzene
c. an isomer of trimethyl benzene

blogfast25 - 1-9-2016 at 13:10

NC#5

A sample is composed of potassium alum, sodium chloride and potassium nitrate. A 30.0 g sample is fully dissolved in water. To the solution 0.300 mole of ammonia (NH₃) is added and a precipitate is formed. The solution is then filtered and the filter cake washed. Filtrate and wash water are quantitatively recovered and the combination diluted to 250.0 ml.

Several 25.0 ml aliquots of this stock solution are titrated with 0.9915 M hydrochloric acid in an acid/base titration. A significant average of 17.4 ml of titrant solution is used.

Questions:

1. Write the ionic reaction equation of the precipitation reaction, state symbols included.

2 points.

2. What is the weight percentage of potassium alum in the sample?

8 points.

[Edited on 2-9-2016 by blogfast25]

blogfast25 - 1-9-2016 at 13:22

NC#6

Production of aluminium:

In one run 20.0 kg of Al is produced by electrolysis of an alumina/crylolite bath. The reaction is:

Al₂O₃(l) == > 2 Al(l) + 3/2 O₂(g)

Due to the heat the carbon anode is partly oxidised by:

C(s) + O₂(g) === > CO₂(g)

After the run it's established the anode has lost 1.2 kg in mass.

The CO₂ and O₂ are separated.

Question:

How much oxygen (in kg) was obtained during the run and after separation from the CO₂?

10 points.

[Edited on 1-9-2016 by blogfast25]

NEMO-Chemistry - 1-9-2016 at 14:17

The deadline for my question has passed, so I guess I ought to post the solution.

3.75 g of a metal chloride is dissolved in water and treated with excess aqueous silver nitrate, resulting in the formation of 10.66 g of precipitate.

Net ionic equation: Ag⁺(aq) + Cl^-(aq) --> AgCl(s)

We obtained 10.66 g of silver chloride, which is 74.37 mmol. *If* the metal was monovalent, that would mean the molar mass of the metal chloride was 3.75 g / 0.07437 mol = 50.4 g/mol, which would give the metal a molar mass of 15.0 g/mol. This is not the molar mass of a metal.

If it's divalent, then the molar mass of the metal chloride would be 3.75 g / (0.07437 mol/ 2) = 100.8 g/mol. Subtracting the two chlorides gives 29.9 g/mol. This is between silicon and phosphorus, which are not metals.

Trivalent gives 44.9 g/mol, which is within experimental error for scandium (which is indeed trivalent).

Tetravalent gives 59.9 g/mol, which is close to cobalt or nickel, neither of which form tetrachlorides.

Pentavalent gives 74.9 g/mol, which would be arsenic, but arsenic is not a metal, and its pentachloride is not ionic.

Hexavalent would give 89.8 g/mol, which is close to yttrium and zirconium, neither of which have a +6 oxidation state. Similarly, rhodium and palladium do not form heptachlorides, and tin doesn't form an octachloride. So the metal must be scandium.

[Edited on 1-9-2016 by DraconicAcid]

Ouch! That was way above my current level, i didnt get close

.

Onwards and upwards! More reading and studying needed then a crack at the other questions.

j_sum1 - 1-9-2016 at 16:28

I like the look of these new problems bloggers. I am going to have a bit of fun doing them.
And, DraconicAcid, I thought the scandium chloride was genius. What a great way to pose a puzzle.

NEMO-Chemistry - 1-9-2016 at 17:03

All of this is an exceptional way to learn. I couldnt arrange enough time to do the last one, but having seen the answer i know i wouldnt have got it.

But learning is about being stretched, so i have learnt yet another way to approach a problem. While doing experiments is alot of fun, this kind of thing is really useful in learning the basics, following a recipe in a book is mainly just practising technique.

Lefaucheux10 - 2-9-2016 at 01:33

NC#5

A sample is composed of potassium alum, sodium chloride and potassium nitrate. A 30.0 g sample is fully dissolved in water. To the solution 0.300 mole of ammonia (NH₃) is added and a precipitate is formed. The solution is then filtered and the filter cake washed. Filtrate and wash water are quantitatively recovered and the combination diluted to 250.0 ml.

Several 25.0 ml aliquots of this stock solution are titrated with 0.9915 M hydrochloric acid in an acid/base titration. A significant average of 17.4 ml of titrant solution is used.

Questions:

1. Write the ionic reaction equation of the precipitation reaction, state symbols included.

2 points.

2. What is the weight percentage of potassium alum in the sample?

8 points.

[Edited on 2-9-2016 by blogfast25]

Questions if you can respond without denaturating the problem :

1) is the alum anh ? or 12 H2O

2) is the NH3 added sufficiant to produce all the precipitate possible and not dissolve any of it ?

THX if informationS can be delivered

THX also if you can't ^^

blogfast25 - 2-9-2016 at 04:58

Quote: Originally posted by Lefaucheux10

Questions if you can respond without denaturating the problem :

1) is the alum anh ? or 12 H2O

2) is the NH3 added sufficiant to produce all the precipitate possible and not dissolve any of it ?

1) potassium alum is 12H2O.

2) NH3(aq) is too weak a base to dissolve the precipitate.

Neme - 2-9-2016 at 12:49

Very nice teasers, I'd appreciate you to continue posting them even after the end of competition. Some of them are very fun to solve

NEMO-Chemistry - 2-9-2016 at 13:26

Quote: Originally posted by Neme

Very nice teasers, I'd appreciate you to continue posting them even after the end of competition. Some of them are very fun to solve

I think alot of people would, they are also great for learning......BUT i suspect the problem is time, it must take a fair bit of time and effort to come up with decent questions.

Its a bit much to expect people to put a huge amount of work in like this, on the other hand a few of the other Guru's might step in after the comp and continue a non prize question thread.

What about the real noob questions like.......Why dont you use litmus paper to test the pH of conc sulphuric acid?

I cant be the only one who has tried it??

[Edited on 2-9-2016 by NEMO-Chemistry]

aga - 2-9-2016 at 13:47

What about the real noob questions like.......Why dont you use litmus paper to test the pH of conc sulphuric acid?

What ?! You put litmus paper in conc sulph !?!?

Changes to Red(acid) then Black(burnt) then Little Bits(burnt to buggery)

These questions are a bit hard for a complete noob - i'm certainly getting stretched a Lot trying to work them out.

Sadly us wormlings cannot see how simple and easy they probably are to those who fly high above us (in terms of chem knowledge).

Maths is very much involved in Chemistry - it's fundamentally necessary to work anything out.

Maybe make a Noob thread with noobs making the noob questions ?

Hardest thing will be where to pitch the level of the questions.

This must be a continual problem for teachers.

NEMO-Chemistry - 2-9-2016 at 14:05

What about the real noob questions like.......Why dont you use litmus paper to test the pH of conc sulphuric acid?

What ?! You put litmus paper in conc sulph !?!?

Changes to Red(acid) then Black(burnt) then Little Bits(burnt to buggery)

Yes i think your right, really difficult to make a quiz for all people. But as long as a reasonable time is given then you have a chance to read up on a concept and try and apply it.

Chemistry is a steep curve for sure, but each little win and snippet of hard won knowledge is well worth the effort needed.

In class i am up against mostly people who did 2 years of lower science, they have the advantage at the moment.

But i noticed a few just do the set work at school and then leave it at that. Few bother doing science at home, even a few who say they want to study the Higher level and then uni.

So while my maths isnt as good as theirs, and my chemistry is currently below theirs. I still think i have an advantage, i try and research everything to death.

I also try things out at home, mostly it goes wrong or is just basic things. But keeps me happy and sooner or later especially in practicals i will have a better technique than the clever people. Well thats the plan anyway..

Glad your being stretched, shakes your confidence a bit when so many say they find the questions easy and are noobs. I find them far from easy but nothing worth having is won easy.

Funny how it seems to be three main types, the ultra competitive who just want to win, then the want to wins for the prize and finally those who just want to do it to learn regardless of any incentive.

Yeah i assumed every noob tried testing the pH of Sulphuric with litmus.

It dawns on you pretty quickly how stupid the idea is, and generally isnt something that should be admitted.

[Edited on 2-9-2016 by NEMO-Chemistry]

aga - 2-9-2016 at 14:31

Been doing Chemistry as a hobby 2.5 years roughly, so still pretty much a noob.

These questions have taken me HOURS of thought, scribbling and research.

Exercises like these definitely stretch the understanding, and you come across information (during research) that you might not encounter any other way.

What person, armed with conc sulphuric acid and litmus paper would NOT stick it in there to see what happens ?

answer: someone who isn't interested in finding stuff out.

For the Record, i do not want the prize - better a noober-noob gets it instead.

The fun is all in the learning and i got a lab already.

[Edited on 2-9-2016 by aga]

Lefaucheux10 - 4-9-2016 at 07:57

I have finished all the problems wait for the results now ^^

thanks guys

MeshPL - 6-9-2016 at 10:43

Will there be more questions posted? I must admit I'm kinda looking forward to them. The difficulty was ok so far, although the amount of quesrptions could be higher...

aga - 6-9-2016 at 11:20

I guess that anyone could post a question.

Maybe best to U2U it to bloggers for QA before posting.

NEMO-Chemistry - 6-9-2016 at 11:52

Quote: Originally posted by MeshPL

Will there be more questions posted? I must admit I'm kinda looking forward to them. The difficulty was ok so far, although the amount of quesrptions could be higher...

Some people are never happy! Like i said before just be grateful people are taking ALOT of time and effort to do this, it just sounds a bit ungrateful and a bit of a cheek to just EXPECT others to spend their time making you happy.

If your getting through them that quickly then you got to wonder if you fall under the noob category. I have no idea what level the original aim was except 'noob'.

Aga i wouldnt call a noob as such, he is pretty clued up on a fair few things and found some of them a challenge. I am not finding them easy and i am having to put alot of work and effort in.

Not trying to be funny with you but it worries me that people pestering will put an end to what is an excellent resource and way to learn.

aga - 6-9-2016 at 12:06

Oh, definitely a Noob.

Yes, ok, done many things so far, yet Understanding WTF i did, why it worked/failed is quite another matter.

Asking for more Free stuff needs to be done nicely, seeing as it's for Free.

As i said, perhaps others might submit a question or two to the QA department.

I just did, albeit of lower quality/difficulty and of a sneaky nature.

[Edited on 6-9-2016 by aga]

NEMO-Chemistry - 6-9-2016 at 12:21

Oh, definitely a Noob.

Yes, ok, done many things so far, yet Understanding WTF i did, why it worked/failed is quite another matter.

Asking for more Free stuff needs to be done nicely, seeing as it's for Free.

As i said, perhaps others might submit a question or two to the QA department.

I just did, albeit of lower quality/difficulty and of a sneaky nature.

[Edited on 6-9-2016 by aga]

Blogfast is the sneaky question master lol. Yes i agree 100%, seeing as its free and has a prize attached then manners are required when asking!

blogfast25 - 6-9-2016 at 13:43

Quote: Originally posted by MeshPL

Will there be more questions posted? I must admit I'm kinda looking forward to them. The difficulty was ok so far, although the amount of quesrptions could be higher...

At least one more coming up, maybe two.

blogfast25 - 11-9-2016 at 02:52

Reminder to Draconic Acid: I haven't received your marks yet!

blogfast25 - 11-9-2016 at 03:07

NC#7

Analysis: method of Standard Addition

An analytical method yields a signal S (in mV) that is directly proportional to the concentration C_R of a reagent R:

$$S=kC_R$$

A sample solution of R of unknown concentration is analysed and yields S = 60 mV.

To 250.0 ml of the solution, 0.05 moles of R and added and the new solution homogenised (assume no significant volume change happens). Analysis of this solution give a signal strength of 120 mV.

Question:

What was the concentration of the original sample solution?

10 points.

[Edited on 11-9-2016 by blogfast25]

blogfast25 - 11-9-2016 at 03:30

NC#8

Reaction rate:

750 ml of 0.500 M HCl(aq) is reacted with a large excess zinc metal acc.:

$$Zn(s) + 2 HCl(aq) \to Zn^{2+}(aq) + 2 Cl^-(aq) + H_2(g)$$

The reaction is carried out with full stirring and at constant temperature. At the start of the experiment the chemist measures the rate of hydrogen evolution to be 20 mmole/s (20 x 10^-3 mol/s).

After some time t the chemist determines that 0.192 moles of HCl have reacted away. No significant change in volume has occurred.

Question:

What is the expected rate of hydrogen evolution at time t?

10 points.

Hint: the reaction rate v obeys the simple law:

$$v=k[HCl]$$

where k is a rate constant.

[Edited on 11-9-2016 by blogfast25]

ficolas - 11-9-2016 at 05:13

They are getting harder D:

aga - 11-9-2016 at 08:14

They are getting harder D:

Great questions, and excellent excercise !

NEMO-Chemistry - 11-9-2016 at 11:40