[darkSun]

Hey guys, I'm an AP chem student studying for the US national chem olympiad. These questions are from the national test, and I just don't understand some of them. And if any of you out there have some experience with the olympiad, anything you have to say about it would be greatly appreciated. Thanks!

Ok, first is Which salt is significantly more soluble in a strong acid than in water?
PbF2, PbCl2, PbBr2, or PbI2.

The answer is PbF2, but what about lead II fluoride makes it more soluble in acids?

From another question, why does mixing equal volumes of .1M HCl and .2M NH3 create a buffer solution? I know mixing weak acids or weak bases with their conjugate salts makes buffer solutions, but this is totally different. And why is the HCl .1M, and the NH3 .2M?

Hmm, I don't know if i should ask this many questions in one topic. Anyway,
For which compound does the reaction
MCO3 >>> MO (s) + CO2 (g) occur most readily?

BeCO3, MgCO3, CaCO3, BaCO3

I had no clue. I thought BaCO3 might be the answer because it has the weakest crystal lattice force, but the answer is BeCO3. ??

Thanks again!

[Borek]

HF - strong acid or a weak one?

What is a definition of buffer? What will be in the solution after you mix given amounts of ammonia and hydrochloric acid? Will it fit buffer definition?

[darkSun]

x] ok I see the reason for the PbF2, H+ attracts the F- ion

So for the buffer, in solution, some of the NH3 combines with H+ to make NH4+. So the buffer solution could be considered to consist of NH3, a weak base, and NH4Cl, its conjugate acid. Is this correct? If so, why is twice as much ammonia needed as HCl?

I appreciate the help Borek.

[Borek]

I would not call NH₄Cl an acid, think in terms of ions present in the solution.

What will you have in the solution if you mix equimolar amounts of ammonia and HCl? Look at the reaction equation, think about the reaction stoichiometry.

[darkSun]

I meant conjugate salt... but I think you're still saying I'm wrong.

The reaction that would occur is H+ + Cl- + NH3 ---> NH4+ + Cl-
I can get no farther. Is the buffer NH3/NH4+ ?

And I found out about the carbonate question, my chemistry teacher said it was just one of those things you just have to know. Seems like chemistry is full of those things...

[Borek]

I meant conjugate salt... but I think you're still saying I'm wrong.

What I meant was what I wrote - think in terms of ions present. NH₄Cl is a salt, just like NH₄Br or (NH₄)₂SO₄. Each is different but they can be all used for the same purpose of creating ammonia buffer. Why? Because it is NH₄⁺ that counts.

The reaction that would occur is H+ + Cl- + NH3 ---> NH4+ + Cl-
I can get no farther. Is the buffer NH3/NH4+ ?

Yep. Buffer should contain pair of conjugated base and acid. Now, assume that the reaction went to completion (ie was stoichiometric). What do you have in solution?

And I found out about the carbonate question, my chemistry teacher said it was just one of those things you just have to know. Seems like chemistry is full of those things...

This is about trends. TBH I am not sure what is the correct answer. I would put my bet on Ba like you did.

[darkSun]

I see. Once the reaction is completed, you have .1 M NH3 and .1 M NH4+. This is the buffer, and there is .2M ammonia to start with so that the amounts of NH3 and NH4+ when the reaction is completed will be the same.

[AWK]

Ok, first is Which salt is significantly more soluble in a strong acid than in water?
PbF2, PbCl2, PbBr2, or PbI2.

The answer is PbF2, but what about lead II fluoride makes it more soluble in acids?

As a rule OK. But just now we met the exception.
PbF2 show different solubility from that expected one (Ksp = 3.7 x 10-8 ).
Hence its solubility will depend greatly on the concetration of strong acid.
Moreover you should use only HNO3 for testing

Hmm, I don't know if i should ask this many questions in one topic. Anyway,
For which compound does the reaction
MCO3 >>> MO (s) + CO2 (g) occur most readily?

BeCO3, MgCO3, CaCO3, BaCO3

http://www.chemguide.co.uk/inorganic/group2/thermstab.html

[darkSun]

What do you mean "only use HNO3" for testing? What's wrong with using other strong acids?

And thanks for the link, its very good.