2010 HSC - Question 18

[clarence.harre]

So, the question is:

Chromate and dichromate ions form an equilibrium according to the following equation.

2CrO4(^2-) + 2H(^+) <--> Cr2O7(^2-) + H2O

Which solution would increase the concentration of the chromate ion (CrO4(^2-)) when added to the equilibrium mixture?

(A) Sodium nitrate
(B) Sodium chloride
(C) Sodium acetate
(D) Ammonium chloride

The answer is C.

My Working:

To get more Chromate ion (i.e. to shift equilibrium to the left) we'd need to add something that changes the concentration on the right side.
From above, that is Cr2O7(^2-) + H2O

A) NaNO3
B) NaCl
C) CH3COONa
D) NH4Cl

I took it that the amount of Na, N and Cl is redundant, as they aren't in the equation, removing option B.
(This is where I got stuck and guessed an answer)

Knowing the answer is C, I now assume that it's because it's the only one that has both H and O, increasing the amount of those two elements on the right side (which doesn't really sit well for my logic at the moment, as I believe it should be in the water format for this to work).

Please point out the flaws in my working, and help me understand why the answer is C

Thanks

[MisterNeo]

Please point out the flaws in my working, and help me understand why the answer is C

Thanks

Hey! This is a tricky one.

From the options, you can see that all the anions of the salts are from strong acids except acetate (from acetic acid).
Usually acetic acid forms an equilibrium in solution due to its weakness.

The equilibrium lies well to the left, thus acetate does not like being an ion and will want to form molecular acetic acid.
When you add acetate into the chromate system, it will bond with H⁺ to become acetic acid, thus reducing its concentration.
Hence, it shifts to the left.

[clarence.harre]

Thank you both for the quick responses.

To clarify, all of them would become their ions in solution, but the acetate is the weakest because it's the biggest (?) and is only monoprotic. It doesn't like being an ion (is it just that one, or do all of them just not like being ions?) so grabs the H+ from the left, forcing the right to make more H+ to replenish it, and consequently makes more of the chromate ion.

[MisterNeo]

Thank you both for the quick responses.

To clarify, all of them would become their ions in solution, but the acetate is the weakest because it's the biggest (?) and is only monoprotic. It doesn't like being an ion (is it just that one, or do all of them just not like being ions?) so grabs the H+ from the left, forcing the right to make more H+ to replenish it, and consequently makes more of the chromate ion.

The equilibrium constant (if you do Industrial chem) determines the ratio of products to reactants in an equilibrium.
Adding acetate into a system with no acetic acid will cause the acetate to bond with the hydrogen in order to achieve that ratio of products to reactants by forming acetic acid.
So since the hydrogen is now not in ion form anymore, the concentration drops and the system shifts to the left to replace the missing hydrogen. And chromate is also produced.

Monoprotic acids aren't all weak.
HCl is a strong acid.

[clarence.harre]

Just want to thank you both for your responses and clarifying my understanding of this.
Turns out, this exact question (5 pt) was in our trial. Thanks again