[tanagholat]

Consider the following aqueous equilibria that take place simultaneously in the same container.


Ca²⁺(aq) + C2O4^2-(aq) <---> CaC2O4(s)

and

H+(aq) + C2O4^2-(aq) <---> HC2O4^-(aq)

Indicate if the amount of CaC2O4(s) increases, decreases, or stays the same as a result of the following disturbances.
Explain each prediction. Assume any volume changes are negligible. Consider the solubility rules.
a) Add LiOH(aq)
b) Add HNO3(aq)
c) add Ca(NO3)2(aq)
d) Increase the volume of the container

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Here are my answers:
a) Increase.  Because the hydroxides will bond with the H+ to form water, and the 2nd rxn will shift to the left in order to rebalance the loss of H+. Then, the excess C2O4- will shift the 1st rxn to the right to form more Ca(C2O4) (s).

b) Decrease.  The HNO3 will cause an increase in H+, so the 2nd rxn will shift to the right (forward) and the decrease in C2O4 2- will cause the 1st rxn to shift to the left in order to return to equilibrium.

c) Increase. The additional Ca will shift the 1st rxn to the right.

d) No change (stays the same). Increase in volume of container does not affect concentration.

I'm relatively confident about c and d. But my (a) and (b) seem a bit out there...help please!

[Borek]

Actually you should rethink answer to d. You are right - concentrations don't change. But it is not concentration that is a problem here, but amount of dissolved substance.

[tanagholat]

I understand what you mean there...but increasing the volume of the container shouldn't affect the amount of dissolved substance either.

Is my actual answer of "no change" wrong?
or was it just my explanation that needs to switch "concentration" with "amount of substance"?

[Borek]

but increasing the volume of the container shouldn't affect the amount of dissolved substance either

So you are saying that 1 mL of 1M solution contains the same amount of substance as 1 L of the same solution?

Although now that I think about it, question is poorly worded. Does changing "volume of the container" mean changing volume of the solution?

[tanagholat]

I understand what you are saying. But i actually do believe my teacher wants to look at it as the "volume of the container" changing.

For the sake of understanding/practice. Lets assume my teacher meant the actual volume of the solution that was changing  and not that volume of the container. If this is the case, then there would be an increase in the amount of CaC2O4...

[Borek]

Lets assume my teacher meant the actual volume of the solution that was changing  and not that volume of the container. If this is the case, then there would be an increase in the amount of CaC2O4...

To repeat myself: so you are saying that 1 mL of 1M solution contains the same amount of substance as 1 L of the same solution?

You have a saturated solution of CaC₂O₄. You increase the volume - concentration stays the same. Does the new solution contain the same amount of dissolved salt?

[tanagholat]

You have a saturated solution of CaC₂O₄. You increase the volume - concentration stays the same. Does the new solution contain the same amount of dissolved salt?
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The amount of dissolved salt would decrease then? Because the equilibrium would shift it to the left since it can become saturated again.

Question: So when doing these equilibrium problems, it is safe to assume they are always saturated unless told otherwise?

Thanks for the help too