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blogfast25
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No, but as I tried to explain, it's a difficult question because there's no single answer. Historically, just to get to the notion that there is an
atomic weight is a serious chunk of scientific history. It's not a simple 'trick' that can be revealed here in 5 minutes.
Here's someone who bravely tries to summarise the history of atomic mass: 'The mess with the mass':
http://h2g2.com/entry/A2911259
[Edited on 24-5-2014 by blogfast25]
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CHRIS25
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Good News
Lessons Learned: Double check your notes!!!!! I had 70 mL of ferric chloride not 125. My last entry in my notes, but alas I continued to evaporate
the solution and forgot to note new volume before storing.
Anyway I titrated with amounts previously written, I added iodide as anhydrous not in a solution.
Results: 56 mLs thiosulphate (I took the measurement the second the blue disappeared, I then added a further 55 mLs of thiosulphate just to see if the
soln would go to clarity and it did not).
So 56 x 0.15 M = 0.0084 (x2) = 0.0168 mol thiosulphate titrated. Then 0.0168 x 0.010 mLs chloride soln = 1.68 M Fe3+ in 70 mLs of chloride
solution.
But this begs the question that this means: 1.68 M x 0.07 L = 0.12 mols of Ferric (not ferrous) in solution. I added 29g 0.5 mol originally. Am I
missing something here or is my titration still way up the creek? The ferric is well and truly oxidized without peroxide, it is a deep yellow against
the sides of the glass but otherwise to look at a very deep brown/black. So there can not be much ferrous left surely?
[Edited on 25-5-2014 by CHRIS25]
‘Calcination… is such a Separation of Bodies by Fire, as makes ‘em easily reducible into Powder; and for that reason ‘tis call’d by some
Chymical Pulverization.’ (John Friend, Chymical Lectures London, 1712)
Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. (William Penn 1644-1718)
The very nature of Random, Chance development precludes the existence of Order - strange that our organic and inorganic world is so well defined by
precision and law. (me)
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aga
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That helps enormously.
thankyou.
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blogfast25
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Chris:
If I understand well you now took 10 ml (not 'mLs', which could stand for 'minor Luddite syndrome' but not much else ; -) ) and titrated that directly
w/o any dilution? A minor problem with that is that the solution may be a bit too acidic for a thiosulphate titration but we'll assume it isn't. But
it is the drawback of not diluting before titration. Maybe you did add some water to the 10 ml original sample?
So you titrated in 56 ml of thiosulphate 0.15 M to end-point (yes, the first disappearance of the blue is your end-point), which is 0.056 L x 0.15
mol/L = 0.0084 mol thiosulphate, equivalent to 0.0168 mol Fe<sup>3+</sup>. Contained in 10 ml that gives 0.0168 mol / 0.010 L = 1.68 M.
I think your titration is correct (but w/o any error analysis). The missing Fe remains a bit of a mystery. Considering the huge error margin with the
thiosulphate solution in the first attempt, both results are roughly in line: your ferric chloride solution is definitely NOT 3 - 4 M.
Now here's an important point: you complain about turbidity (non-clarity). When does this first appear? Was the original sample clear? Does the
turbidity occur during titration and what does it look like?
Another point is the use of 56 ml of titrant solution: that is a HUGE volume by most titrators standards. That is why we usually dilute the original
sample and then only titrate a smaller, known volume of it. Are you using syringes for the injection of whales or elephants? ;-) Multiple additions
of full syringes also introduces error...
[Edited on 25-5-2014 by blogfast25]
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CHRIS25
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No, I diluted with 100 mLs of water (mLs was watson fawkes correction of my very first postings where I used Mls back then). Anyway I added the
titrant to the whole lot. Also I used the burette now that I have one. The missing iron is a mystery, really who pinched it? So, the turbidity, it
was opaque, you could quite clearly see through the titrated soln, it was simply not the clarity of water, it had an extremely subtle blue silvery
tint, now this has to be something to do with taking too long? After completion, I left the titration soln to stand just to observe a thick milky
white turbidity develop within about 5 minutes. I have to read up on this because I know I have read an explanation somewhere but can not remember.
I do know that the whole titration has to be done quickly, but this time I took more time with the whole process, probably about 10 minutes.
[Edited on 25-5-2014 by CHRIS25]
Sorry - to answer another question, the original sample was a nice bright yellow as expected with dilution and the colour of ferric ions in this.
[Edited on 25-5-2014 by CHRIS25]
Darn it, that 3 M approximation was in actual fact acid reserve as I go through my notes, not Molarity of Fe. I do admit confusion here, but I
originally began with 1.9 M HCl (and increased it) and 0.5 mol Fe filings. What exactly am I measuring? it is the amount of oxidised Fe in soln. So
if it is 1.68M = 0.12 mol of Fe3+ then that surely must mean that the soln has a long way to go before the ferrous oxidises? I can not
logically deduce it any other way based on the fact that Iron simply can not evaporate.
[Edited on 25-5-2014 by CHRIS25]
[Edited on 25-5-2014 by CHRIS25]
‘Calcination… is such a Separation of Bodies by Fire, as makes ‘em easily reducible into Powder; and for that reason ‘tis call’d by some
Chymical Pulverization.’ (John Friend, Chymical Lectures London, 1712)
Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. (William Penn 1644-1718)
The very nature of Random, Chance development precludes the existence of Order - strange that our organic and inorganic world is so well defined by
precision and law. (me)
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blogfast25
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To be entirely SC ('scientifically correct') a ml is one cm<sup>3</sup> ('one cubic centimeter') and 1 L should be one
dm<sup>3</sup> ('one cubic decimeter') but unfortunately that hasn't caught on so much and old habits die hard.
The milky white turbidity is elemental sulphur but it can only arise from excess thiosulphate in acid conditions, unless the S4O6(2-) reaction product
also shows that behaviour. Must look into that...
If you started from 0.5 mol Fe filings, and assuming no loss of iron and all iron as Fe<sup>3+</sup> then estimating the
Fe<sup>3+</sup> concentration is only a mater of knowing the final volume of the solution after all your manipulations:
[Fe<sup>3+</sup>] = mol Fe<sup>3+</sup> / Final Volume (in L), so go back to your notes.
You're using the burette: serious progress!
But 10 min is quite a long time, there's a risk of some of the iodide being oxidised by air, back to iodine. 2 - 3 mins per titration, tops.
[Edited on 25-5-2014 by blogfast25]
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CHRIS25
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Yes I did that this morning = 7 M. So what does that say about this titration then? Where exactly is the point in this, What does it say? Why the
huge discrepancy, I do not understand - times like this when you wish you were in college.
unless the S4O6(2-) reaction product also shows that behaviour.
will look into this also
[Edited on 25-5-2014 by CHRIS25]
‘Calcination… is such a Separation of Bodies by Fire, as makes ‘em easily reducible into Powder; and for that reason ‘tis call’d by some
Chymical Pulverization.’ (John Friend, Chymical Lectures London, 1712)
Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. (William Penn 1644-1718)
The very nature of Random, Chance development precludes the existence of Order - strange that our organic and inorganic world is so well defined by
precision and law. (me)
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blogfast25
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Quote: Originally posted by CHRIS25  | Yes I did that this morning = 7 M. So what does that say about this titration then? Where exactly is the point in this, What does it say? Why the
huge discrepancy, I do not understand - times like this when you wish you were in college.
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7 M would mean a final volume of about 71 ml: 0.5 mol / 0.071 L = 7 M. That sounds waaaayyy too high to me. 7 M is 1096 g
FeCl<sub>3</sub> / L !!!! The solubility limit of FeCl3.6H2O is only 918 g/L. Such solutions would be very, very dark, almost black (I
have a saturated solution of FeCl3 in my lab: it's like black syrup!)
Such high concentrations could only be achieved by painstaking evaporation of a more dilute solution: they CANNOT be achieved directly by dissolving
iron in acid.
Your titrations are obviously a bit flawed but not that much.
Something is wrong at the level of your concentration estimate. Either you used much less iron than you seem to remember or something like that.
To be able to titrate a 7 M FeCl3 solution with a 0.15 M thiosulphate solution (via iodide, of course), you would have to dilute the sample 25 times,
for instance 10 ml sample in 250 ml water. Then take 20.0 ml of that dilution, add iodide and titrate with 0.15 M sodium
thiosulphate. That should give you an end point AROUND 20 ml.
[Edited on 25-5-2014 by blogfast25]
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blogfast25
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Just looking at the amount of KI needed.
Assuming you're right about the approx. 7 M, you took 10 ml = 0.01 L of that, so it contains 0.07 mol Fe3+.
Acc. Fe3+ + I- === > Fe2+ + 1/2 I2
... you need to add AT LEAST 0.07 mol iodide or 0.07 mol x 166 g/mol = 11.6 g KI. Anything less and you'll get a faulty (too low)
reading. But such a titration would require a whopping 230 - 240 ml of 0.15 M sodium thiosulphate to reach end point!!
[Edited on 25-5-2014 by blogfast25]
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CHRIS25
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Right. Determination here to sort this out. My notes and my memory agree, 28 grams iron, then two filtrations and another 2 grams iron due to a
little loss during both filterings so I added a gram each time. half a mol in 125 mLs HCl and then a further 40 mLs later on. This is just a
summary. I know it was 28g because I need that amount for something else. So another titration, different and larger measurements - BIG ones and we
will see what happens this time.
‘Calcination… is such a Separation of Bodies by Fire, as makes ‘em easily reducible into Powder; and for that reason ‘tis call’d by some
Chymical Pulverization.’ (John Friend, Chymical Lectures London, 1712)
Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. (William Penn 1644-1718)
The very nature of Random, Chance development precludes the existence of Order - strange that our organic and inorganic world is so well defined by
precision and law. (me)
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blogfast25
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No, don't do BIG ones, it won't help much.
Try what I wrote above.
Take a 250 ml beaker and put in 10.0 ml of the concentrated solution. Fill up to the 250 ml mark and stir well.
Now transfer 20.0 ml of that dilution into your conical flask, adding another 50 ml or so of water. Add 1.2 g of KI and allow to dissolve and react
for a few minutes, constantly swirling that flask.
Titrate this with 0.15 M thiosulphate from your burette, adding starch towards the end. The end point should be in the region of 17 to 22 ml of 0.15 M
thiosulphate, based on 7 M Fe3+ in the original sample. See what you find and we'll calculate the result later.
Determination? That's what we like to hear!
[Edited on 25-5-2014 by blogfast25]
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CHRIS25
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Three titrations done
Sorry took so long, had 11 hours and a 585 km round trip yesterday so too tired, this morning re-freshed...
Firstly Gert, I can not work out mathematically how to calculate the dilution factors into the stoichiometry so please show me (I mean only from no.1
titration, the last two I can do). I can do maths so long as there are examples but can not work out mathematical concepts to save my life. So here
are the results from 3 titrations (complicated by dilutions):
1. Extracted 20 mLs Ferric Chloride from 250 mLs water (this was 20 mLs added to 240 so a dilution factor of 24); Then added 50 mLs to the 20 mL
sample and titrated. endpoint at 3.5/4.0 mLs thiosulphate. (this reading is suspect because endpoint reached far too quickly and starch did not turn
soln blue so I was passed the endpoint, I did not shake the flask quick enough as it dripped in).
Lesson learned carry on....
2. 30 mLs ferric chloride extracted from same diluted 250 mL solution. NO extra water added this time. 5.5 mLs thiosulphate reached endpoint.
3. Same again, 30 mLs ferric chloride extracted and 5.3 mLs thiosulphate to reach endpoint.
As I said I am thick at maths without first an example to follow when it gets to diluting now. So your explanation would be extremely appreciated.
Just so that you are clear I used 1.2 g iodide, 0.15M thiosulphate and the dilutions that you asked only on the first titration, second and third
titrations were as you said, except without the extra 50 mLs water After that I had extracted the ferric sample from the 250mLs (10 mLs ferric
chloride + 240 mLs water)
[Edited on 27-5-2014 by CHRIS25]
[Edited on 27-5-2014 by CHRIS25]
‘Calcination… is such a Separation of Bodies by Fire, as makes ‘em easily reducible into Powder; and for that reason ‘tis call’d by some
Chymical Pulverization.’ (John Friend, Chymical Lectures London, 1712)
Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. (William Penn 1644-1718)
The very nature of Random, Chance development precludes the existence of Order - strange that our organic and inorganic world is so well defined by
precision and law. (me)
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blogfast25
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Chris:
”1. Extracted 20 mLs Ferric Chloride from 250 mLs water (this was 20 mLs added to 240 so a dilution factor of 24); Then added 50 mLs to the 20
mL sample and titrated. endpoint at 3.5/4.0 mLs thiosulphate. (this reading is suspect because endpoint reached far too quickly and starch did not
turn soln blue so I was passed the endpoint, I did not shake the flask quick enough as it dripped in).”
This is confusingly put or maybe I’m easily confused. 
I take it to mean this. You took 20 ml of original sample and added it to 240 ml water. That would make 260 ml and a dilution factor of 260 / 20 = 13.
Of this dilution you took 20 ml and added 50 ml water and titrated this with 3.5 ml thiosulphate 0.15 M. That is 0.0035 L x 0.15 mol/L = 0.000525 mol
thiosulphate or 0.00105 mol Fe<sup>3+</sup>, contained in 20 ml of DILUTION, so the dilution is 0.00105 mol / 0.02 L = 0.0525 M.
You diluted by a factor of 13, so the original solution is 13 x 0.0525 = 0.6825 M.
For titration no2 (30 ml of dilution, 5.5 ml thiosulphate used) we get: 0.0055 x 0.15 = 0.000825 mol thiosulphate = 0.00165 mol Fe3+, in 30 ml
dilution or 0.00165 / 0.030 = 0.055 M. The original solution: 0.055 x 13 = 0.715 M.
The KI amounts are correct, yet nothing tallies!
Are you sure your sodium thiosulphate solution is 0.15 M and not 1.5 M? For 1.5 M things would work out at 7 M, as you suspect.
To prepare for instance 200 ml (that is 200/1000 = 0.2 L) of 0.15 M thiosulphate from solid sodium thiosulphate pentahydrate (molar mass 248.18
g/mol) you need to weigh:
0.2 L x 0.15 mol/l x 248.18 g/mol = 7.44 g and dissolve it in 200 ml of water. Are you sure this is what you did?
And then there's this bit:
"After that I had extracted the ferric sample from the 250mLs (10 mLs ferric chloride + 240 mLs water)"
... which hopelessly contradicts what you wrote above. Chris, which is it? 10 ml ferric chloride + 240 ml of water or 20 ml ferric chloride + 240 ml
of water? BIG difference: dilution factors of respectively 25 and 13! But even that doesn't explain what is going on... For No2 with dilution factor
25 would give 1.37 M for the ferric chloride solution.
[Edited on 27-5-2014 by blogfast25]
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CHRIS25
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Ok. I guess I am working at too many tasks at once. Sorry. please allow me to start again. The following is what I used, I did what you advised
for the first titration. Yes I placed 7.44g of thiosulphate made up to 200 mLs water. 1.2g iodide each titration. The ferric chloride (my typo
above - so sorry). I took 10 mLs from the original ferric chloride solution, put into beaker and added 240 mLs water.
Titration 1: Took 20 mLs from this beaker, placed it into the flask under the burette and added 50 mLs water and titrated it with 0.15 M
thiosulphate.
Titration 2: Took 30 mLs from this same beaker, placed it into the flask (NO EXTRA WATER ADDED), titrated with 0.15 thiosulphate.
Titration 3: Exactly the same as Titration number 2.
I do apologize for the typo. The thiosulphate endpoints are as written all correct.
My calculations are now working out at between 12.6 M and 13.2 M giving me 0.82 and 0.86 mols of Iron respectively. Going from too little to too
much.
Lesson learned, concentrated solutions are inacurate when titrating? These results appear a more accurate reflection, but can not account for the
extra.
[Edited on 27-5-2014 by CHRIS25]
[Edited on 27-5-2014 by CHRIS25]
‘Calcination… is such a Separation of Bodies by Fire, as makes ‘em easily reducible into Powder; and for that reason ‘tis call’d by some
Chymical Pulverization.’ (John Friend, Chymical Lectures London, 1712)
Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. (William Penn 1644-1718)
The very nature of Random, Chance development precludes the existence of Order - strange that our organic and inorganic world is so well defined by
precision and law. (me)
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blogfast25
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Chris:
"My calculations are now working out at between 12.6 M and 13.2 M giving me 0.82 and 0.86 mols of Iron respectively. Going from too little to too
much."
How do you arrive at these numbers? Because I don't. As I wrote above, I expected about 20 ml thiosulphate for 20 ml of dilution and you're barely
getting 5 ml! Something is still very wrong, trust me...
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CHRIS25
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It seems my calculator can't do maths either....
Ok, concentrating on titration number 2 only:
0.0055 L x 0.15 M = 0.000825
0.000825 x 2 = 0.00165
0.00165/0.030 L sample taken = 0.055
0.055 x 24 (dilution factor) = 1.32 M
1.32 M = 0.0858 mols Fe.
If this is all wrong then I am thickly stupid and will drink my ferric chloride for dinner.
Interestingly these figures mathch the very first titrations that I did, where I had a 1.3M. Something is wrong, and I will make another solution of
FeCl.
[Edited on 28-5-2014 by CHRIS25]
[Edited on 28-5-2014 by CHRIS25]
[Edited on 28-5-2014 by CHRIS25]
‘Calcination… is such a Separation of Bodies by Fire, as makes ‘em easily reducible into Powder; and for that reason ‘tis call’d by some
Chymical Pulverization.’ (John Friend, Chymical Lectures London, 1712)
Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. (William Penn 1644-1718)
The very nature of Random, Chance development precludes the existence of Order - strange that our organic and inorganic world is so well defined by
precision and law. (me)
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blogfast25
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You are careless around numbers, young man. Line 4 of your calculation: the dilution factor is 25, NOT 24.
The similarity with your earlier results struck me too.
Would you mind sending me some of your CURRENT solution? I need to find some rest: by analysing your sample myself, I might restore my biorhythm!
Seriously.
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CHRIS25
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I used 24 on purpose, I thought that 10 mLs ferric soln with 24 times the water = 250 mLs total.
‘Calcination… is such a Separation of Bodies by Fire, as makes ‘em easily reducible into Powder; and for that reason ‘tis call’d by some
Chymical Pulverization.’ (John Friend, Chymical Lectures London, 1712)
Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. (William Penn 1644-1718)
The very nature of Random, Chance development precludes the existence of Order - strange that our organic and inorganic world is so well defined by
precision and law. (me)
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DraconicAcid
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Now you're being thick. The dilution factor is the ratio of the final volume of the solution to the initial volume of the solution, not the amount of
water you've added. If you had taken 1 mL of your solution, and diluted it with 1 mL of water, would you say that that the concentration should
change by a factor of 2 (i.e., half of what it was), or a factor of 1 (i.e., stay the same)?
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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blogfast25
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Yes, but that makes the dilution factor = Final Volume / Initial Volume = 250 / 10 = 25. Important, that...
You can also do a density measurement on the ferric chloride, that is a non-destructive measurement and accurate enough to distinguish
between say 1.5 M and 7 M FeCl3.
Method, see here:
http://www.sciencemadness.org/talk/viewthread.php?tid=30274&...
[Edited on 28-5-2014 by blogfast25]
[Edited on 28-5-2014 by blogfast25]
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CHRIS25
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Ok, (156.5 - 95.3)(142.5 - 95.3) = 61.2 x 47.2
I understand density and SG, but have no idea what I am supposed to do with these figures. Sorry. Have not learned this technique yet. Sorry forgot
- 50 mLs this was.
[Edited on 28-5-2014 by CHRIS25]Hold on, is tis 1.29 SG then?
[Edited on 28-5-2014 by CHRIS25]
‘Calcination… is such a Separation of Bodies by Fire, as makes ‘em easily reducible into Powder; and for that reason ‘tis call’d by some
Chymical Pulverization.’ (John Friend, Chymical Lectures London, 1712)
Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. (William Penn 1644-1718)
The very nature of Random, Chance development precludes the existence of Order - strange that our organic and inorganic world is so well defined by
precision and law. (me)
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blogfast25
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| Quote: Originally posted by CHRIS25 | Ok, (156.5 - 95.3)(142.5 - 95.3) = 61.2 x 47.2
I understand density and SG, but have no idea what I am supposed to do with these figures. Sorry. Have not learned this technique yet. Sorry forgot
- 50 mLs this was.
[Edited on 28-5-2014 by CHRIS25]Hold on, is tis 1.29 SG then?
[Edited on 28-5-2014 by CHRIS25] |
Error alert: (156.5 - 95.3) / (142.5 - 95.3) = 61.2 / 47.2 = 1.30 (1.2966 = 1.30)
Hold on while I calculate the theor. density for FeCl3 7 M.
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blogfast25
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Ok, making a few reasonable (but not necessarily entirely correct) assumptions, I get a theoretical density for 7 M FeCl3 of 1.74
g/cm<sup>3</sup>, quite far removed from 1.30 g/cm<sup>3</sup>, confirming my suspicion that this solution is quite
concentrated but not nearly 7 mol/L.
Since as FeCk3 has a much higher density (2.898) than water (1), the higher the concentration of FeCl3, the higher the density of the solution.
[Edited on 28-5-2014 by blogfast25]
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CHRIS25
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I have just been searching the web, I have no notes on this one. How do you convert this density into the Molarity?
‘Calcination… is such a Separation of Bodies by Fire, as makes ‘em easily reducible into Powder; and for that reason ‘tis call’d by some
Chymical Pulverization.’ (John Friend, Chymical Lectures London, 1712)
Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. (William Penn 1644-1718)
The very nature of Random, Chance development precludes the existence of Order - strange that our organic and inorganic world is so well defined by
precision and law. (me)
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DraconicAcid
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The CRC Handbook gives a density of 1.32 g/mL for 32% FeCl3, which is 2.59 M.
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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