Sciencemadness Discussion Board - Identifying Unknown White Solid Compounds in a Lab

Identifying Unknown White Solid Compounds in a Lab

a.dunn.wa - 13-11-2012 at 12:00

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.

tetrahedron - 13-11-2012 at 12:55

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]

a.dunn.wa - 13-11-2012 at 13:39

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

phlogiston - 13-11-2012 at 14:43

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]

zed - 13-11-2012 at 18:57

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]

sargent1015 - 13-11-2012 at 20:45

Look up the brown ring test for nitrate ions, very cool when it works!

bfesser - 14-11-2012 at 09:55

<a href="viewthread.php?tid=21604#pid261000">Taste</a> them? (Seriously; don't.)
<ol type="1"><li>Mg(OH)2 – insoluble in H2O, grey-brown Ag+ precip.</li>
<li>H2sol., white AgCl precip.</li>
<li>Pb(NO3)2 – precip. with H2SO4</li>
<li>CaCO3 – insol. in H2O, effervescence with H2SO4 (CO2↑)</li>
<li>KNO3 – H2O sol., no Ag+ precip., Pb(NO3)2 previously identified</li>
<li>Na3PO4 – H2O sol., green-yellow Ag+ precip.</li>
<li>H2sol., white AgCl precip.</li>
<li>Na2SO3 – H2O sol., H2SO4 no rxn., milky precip. with Ag+, #5 is KNO3, #3 is Pb(NO3)2</li></ol>

Quote: Originally posted by tetrahedron

unknowns 5 and 8 must be KNO3 and Na2SO3

tetrahedron, I took this statement to be correct, but your logic eludes me. Will you please explain your reasoning?

tetrahedron is correct on the ZnCO3 formation when CO2 is lead into a Zn2+ solution. To distinguish Na2SO4 from KNO3, use the previously identified (Unknown 3) Pb(NO3)2, as PbSO4 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 (NH4Cl or ZnCl2), 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.

Na3PO4, NH4Cl, ZnCl2, KNO3, Mg(OH)2, Pb(NO3)2, CaCO3, Na2SO3
↓insoluble in H2O↓
CaCO3, Mg(OH)2
↓H2SO4 effervescence↓
CaCO3

I referenced <a href="http://www.biblio.com/books/159831535.html" target="_blank">Introduction to Semimicro Qualitative Analysis, 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]

Hexavalent - 14-11-2012 at 10:45

Quote: Originally posted by sargent1015

Look up the brown ring test for nitrate ions, very cool when it works!

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.

a.dunn.wa - 14-11-2012 at 12:00

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?

phlogiston - 14-11-2012 at 12:28

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.

tetrahedron - 14-11-2012 at 12:50

phlogiston must be right. i missed silver sulfite's poor solubility.

Quote: Originally posted by bfesser

To distinguish Na2SO4 from KNO3, use the previously identified (Unknown 3) Pb(NO3

2, as PbSO4 will precipitate.

the anion in question is sulfite. however,

Quote:

Solutions exposed to air are eventually oxidized to sodium sulfate.

so your proposed test might work given enough time.

bfesser - 14-11-2012 at 15:36

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 Na2SO3 from KNO3, use the previously identified Pb(NO3

2, as PbSO3 will precipitate.

a.dunn.wa - 14-11-2012 at 17:59

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.

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.

sargent1015 - 14-11-2012 at 20:06

Quote: Originally posted by Hexavalent

Quote: Originally posted by sargent1015

Look up the brown ring test for nitrate ions, very cool when it works!

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.

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]