a.dunn.wa
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Identifying Unknown White Solid Compounds in a Lab
Hello Sciencemadness,
I am seeking advice from you chemistry maestros once again, this time for developing procedure and drawing conclusions on an identification lab.
We were given 8 unknown white substances and the 8 identities, we just have to match them up. The trick is we can only use silver nitrate and sulfuric
acid as reagents (distilled water is also available) to identify them. [AgNO3 is 0.1 M, H2SO4 is 3.0 M]
The 8 identities are:
Na3PO4
NH4Cl
ZnCl2
KNO3
Mg(OH)2
Pb(NO3)2
CaCO3
Na2SO3
We also CANNOT do any flame tests/burn any of the chemicals. So here is what we have so far, and I was wondering what you guys think would be a good
place to go from here.
1. Test for solubility in water
Unknown 1 - insoluble
Unknown 2 - soluble
Unknown 3 - soluble
Unknown 4 - insoluble
Unknown 5 - soluble
Unknown 6 - soluble
Unknown 7 - soluble
Unknown 8 - soluble
2. RXNs w/ AgNO3
Unknown 1 - reacted, created grey-brown precipitate
Unknown 2 - reacted, created white precipitate (assumed to be AgCl2)
Unknown 3 - no rxn
Unknown 4 - no rxn
Unknown 5 - no rxn
Unknown 6 - reaction, green-yellow precipitate
Unknown 7 - reacted, white precipitate (assumed to be silver chloride again)
Unknown 8 - not sure. Milky color initially would fade away after a bit
3. Reactions with sulfuric acid
Unknown 1 - not done
Unknown 2 - not done
Unknown 3 - reaction, solid white precipitate
Unknown 4 - reaction, vigorous fizzling and gas given off
Unknown 5 - no reaction
Unknown 6 - not done
Unknown 7 - not done
Unknown 8 - no reaction
CONCLUSIONS SO FAR:
Unknown 1 - Mg(OH)2 because not soluble in water and the grey-brown precipitate would be silver hydroxide
Unknown 2 - no conclusion
Unknown 3 - Pb(NO3)2 because the only precipitate that should be made w/ sulfuric acid out of the unknowns is the lead compound
Unknown 4 - CaCO3 because not soluble in water and there was no reaction with silver nitrate
Unknown 5 - no conclusion
Unknown 6 - Na3PO4 because soluble in water and rxn w/ silver nitrate made greenish-yellow ppt
Unknown 7 - no conclus.
Unknown 8 - no conlus.
So basically, where should we go from here? Do you guys see any conclusions we could additionally make, or any that are leaps/possibly incorrect?
Also, we have 2 more lab days so if you have any tests you could think of with the materials we have available we are all ears.
Thanks, we truly appreciate your help.
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tetrahedron
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your conclusions seem right so far. unknowns 2 and 7 must be the chlorides. lead some of the CO2 produced from CaCO3 (unknown 4) into each solution to
determine which one is the zinc compound (temporary formation of ZnCO3 precipitate). unknowns 5 and 8 must be KNO3 and Na2SO3. shouldn't the dry
sulfite generate SO2 bubbles upon addition of sulfuric acid?
[Edited on 13-11-2012 by tetrahedron]
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a.dunn.wa
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Quote: Originally posted by tetrahedron  | your conclusions seem right so far. unknowns 2 and 7 must be the chlorides. lead some of the CO2 produced from CaCO3 (unknown 4) into each solution to
determine which one is the zinc compound (temporary formation of ZnCO3 precipitate). unknowns 5 and 8 must be KNO3 and Na2SO3. shouldn't the dry
sulfite generate SO2 bubbles upon addition of sulfuric acid?
[Edited on 13-11-2012 by tetrahedron] |
We will definitely test unknowns 5 and 8 with the sulfuric acid, dry. Interesting idea on the CO2 gas, we will give that a go as well tomorrow and
report out! Thanks
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phlogiston
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I think unknown 8 should be ammonium chloride. The reaction you observed when adding silver nitrate is that a little silver chloride precipitate
initially formed, but the excess of ammonia complexed the silver, forming a soluble silver ammonia complex ion.
Silver sulfite is also nearly insoluble and should give a precipitate, although I am not sure of the color.
My CRC handbook says 'white crystals', whereas this page says 'gray powder':
http://www.scbt.com/datasheet-272467-silver-sulfite.html
sulfite should evolve some SO2 upon acidification, readily detectable by its smell (careful!)
If 8 is NH4Cl, you are left with only 3 unknowns: 2, 5 and 7 and three candidates:ZnCl2, KNO3 and Na2SO3.
Since both the sulfite and ZnCl2 would give a prec with AgNO3, number 5 must be the KNO3 which is the only that doesn't give a precipitate, leaving
only 2 and 7 unknown.
I suggest adding sulfuric acid to both, the one smelling of SO2 is sulfite.
Alternatively, mix both with the sodium phosphate (number 6). Zinc phosphate is insoluble, whereas the sodium sulfite won't give a visible reaction.
[Edited on 13-11-2012 by phlogiston]
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zed
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Hey! Nobody said you can't examine the materials under magnification. Compare the materials appearances.... to Knowns. This is generally
considered kosher. Crystal forms do not lie.
[Edited on 14-11-2012 by zed]
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sargent1015
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Look up the brown ring test for nitrate ions, very cool when it works!
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bfesser
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<a href="viewthread.php?tid=21604#pid261000">Taste</a> them? (Seriously; don't.)
<ol type="1"><li><strong>Mg(OH)<sub>2</sub></strong> – insoluble in H<sub>2</sub>O,
grey-brown Ag<sup>+</sup> precip.</li>
<li>H<sub>2</sub>sol., white AgCl precip.</li>
<li><strong>Pb(NO<sub>3</sub>)</sub>2</sub></strong> – precip. with
H<sub>2</sub>SO<sub>4</sub></li>
<li><strong>CaCO<sub>3</sub></strong> – insol. in H<sub>2</sub>O, effervescence with
H<sub>2</sub>SO<sub>4</sub> (CO<sub>2</sub>↑)</li>
<li><strong>KNO<sub>3</sub></strong> – H<sub>2</sub>O sol., no Ag<sup>+</sup> precip.,
Pb(NO<sub>3</sub>)</sub>2</sub> previously identified</li>
<li><strong>Na<sub>3</sub>PO<sub>4</sub></strong> – H<sub>2</sub>O sol., green-yellow
Ag<sup>+</sup> precip.</li>
<li>H<sub>2</sub>sol., white AgCl precip.</li>
<li><strong>Na<sub>2</sub>SO<sub>3</sub></strong> – H<sub>2</sub>O sol.,
H<sub>2</sub>SO<sub>4</sub> no rxn., milky precip. with Ag<sup>+</sup>, #5 is KNO<sub>3</sub>, #3 is
Pb(NO<sub>3</sub>)<sub>2</sub></li></ol>
tetrahedron, I took this statement to be correct, but your logic eludes me. Will you please explain your reasoning?
tetrahedron is correct on the ZnCO<sub>3</sub> formation when CO<sub>2</sub> is lead into a Zn<sup>2+</sup>
solution. To distinguish Na<sub>2</sub>SO<sub>4</sub> from KNO<sub>3</sub>, use the previously identified
(Unknown 3) Pb(NO<sub>3</sub>)<sub>2</sub>, as PbSO<sub>4</sub> will precipitate.
It's best to lay out your experimental data in tabular form. Don't forget to note things like exothermic/endothermic solvation (does the beaker get
warmer/colder when the solid is dissolved in water), general appearance, odor, etc.. List & cross out identities as you determine unknowns, to
make it easier to think about. As for 2 & 7, it's 50:50 (NH<sub>4</sub>Cl or ZnCl<sub>2</sub>), since you didn't perform
all tests on all unknowns... be more careful next time. Before you do the tests, make a tree flow diagram thingy to figure out what tests you need to
do where to distinguish compounds–similar to below, but with more branches.
Na<sub>3</sub>PO<sub>4</sub>, NH<sub>4</sub>Cl, ZnCl<sub>2</sub>, KNO<sub>3</sub>,
Mg(OH)<sub>2</sub>, Pb(NO<sub>3</sub>)<sub>2</sub>, CaCO<sub>3</sub>,
Na<sub>2</sub>SO<sub>3</sub>
↓insoluble in H<sub>2</sub>O↓
CaCO<sub>3</sub>, Mg(OH)<sub>2</sub>
↓H<sub>2</sub>SO<sub>4</sub> effervescence↓
CaCO<sub>3</sub>
I referenced <a href="http://www.biblio.com/books/159831535.html" target="_blank"><em>Introduction to Semimicro Qualitative
Analysis</em>, 4th ed., by C.H. Sorum</a> while writing this post. I also assumed that no two of the compounds are the same compound.
[Edited on 11/14/12 by bfesser]
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Hexavalent
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This would be great, but if only you'd read what they have available; it requires iron (II) sulfate, which apparently they don't have.
"Success is going from failure to failure without loss of enthusiasm." Winston Churchill
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a.dunn.wa
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Thanks for the tips and advice everyone. After a second day in the lab we have reached the following conclusions:
Unknown 7 is the zinc chloride, as it is deliquescent, we left it out on a paper towel and it clearly left/made water with the atmosphere.
We confirmed #4 is calcium carbonate, as we did see the effervescence w/ sulfuric acid.
Our biggest wonder right now is unknown 8. When we tested it w/ silver nitrate again today, we noticed it would make an initial white precipitate that
would disappear after a bit, but as we add more and more silver nitrate it would stay white. Then we added additional unknonwn #8 to it, and it would
go back to a clear color. We think this is because it is NH4Cl, since the initial ppt is AgCl, the only reason it would disappear is to make an
aqueous coordination compound w/ the ammonium. Thoughts?
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phlogiston
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That is exactly what is happening, as I already explained several posts back, you may have missed it.
1. Mg(OH)2
2. Na2SO3
3. Pb(NO3)2
4. CaCO3
5. KNO3
6. Na3PO4
7. ZnCl2
8. NH4Cl
A) Mix Na3PO4 (6) with 7 and 2 ---> If 7 is indeed ZnCl2 it should give a precipitate, confirming it.
B) mix acid with 2 and 7 ----> 2 should smell of SO2, confirming it is sulfite.
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tetrahedron
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phlogiston must be right. i missed silver sulfite's poor solubility.
| Quote: Originally posted by bfesser | To distinguish Na<sub>2</sub>SO<sub>4</sub> from KNO<sub>3</sub>, use the previously identified (Unknown 3)
Pb(NO<sub>3</sub> <sub>2</sub>, as
PbSO<sub>4</sub> will precipitate. |
the anion in question is sulfite. however,
so your proposed test might work given enough time.
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bfesser
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Yeah, sorry. That post took me hours to write, due to all the subscripts/superscripts... I'm sure that's not the only error I made. Lead sulfite is
insoluble, regardless (CRC Handbook, 89th ed.). That sentence should read as follows:
To distinguish Na<sub>2</sub>SO<sub>3</sub> from KNO<sub>3</sub>, use the previously identified
Pb(NO<sub>3</sub><sub>2</sub>, as
PbSO<sub>3</sub> will precipitate.
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a.dunn.wa
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| Quote: Originally posted by phlogiston | That is exactly what is happening, as I already explained several posts back, you may have missed it.
1. Mg(OH)2
2. Na2SO3
3. Pb(NO3)2
4. CaCO3
5. KNO3
6. Na3PO4
7. ZnCl2
8. NH4Cl
A) Mix Na3PO4 (6) with 7 and 2 ---> If 7 is indeed ZnCl2 it should give a precipitate, confirming it.
B) mix acid with 2 and 7 ----> 2 should smell of SO2, confirming it is sulfite.
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That list is exactly what we had, it's nice to have re-assurance on that. We will def. get A+B done to verify those 2 identities as well; thanks for
all the help.
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sargent1015
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| Quote: Originally posted by Hexavalent |
This would be great, but if only you'd read what they have available; it requires iron (II) sulfate, which apparently they don't have.
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What? It just requires Sulfuric acid and a nitrate to be present. I think it's strong sulfuric though...
Edit: I never did the iron sulfate back in my high school days and the test worked... Weird...
[Edited on 15-11-2012 by sargent1015]
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