[Chung Ling Boys]

my teacher gave us a test with this question:To a solution [Ca²⁺_(aq)  ]=0.00500 mol dm^-3,sufficient solid ammonium oxalate, (NH₄)C₂O₄ is added to make the initial concerntration [C₂O₄^2- ]=0.00510 mol dm^-3.Calculate the equilibrium concerntration of Ca2+ and C₂O₄^2- ions resulting from the precipitation reaction.(Given K_sp of CacC₂O₄=2.70 x 10-9 mol² dm^-6).
*note that ionic qoutient is much larger then K_sp.both ions present in excess.

according to her, we must first assume all the ions precipitate at the amount of the less concerntrated ions(i.e. C₂O₄^2-) ,hence there will be remains of the ions which was more initially. at this point, we must use common ions effect to deal with the remaining steps.

is the method my teacher suggested correct?

[Albert]

It seems reliable    to me, except for the fact that Ca2+ is the less concerntrated ion (according to what you wrote).

[Chung Ling Boys]

the initial concerntartion is given, probably the equilibrium is not achive yet, thats why the concerntration is a bit strange.

I have been discussing the method with my classmates, we were very confused by the fact that we can assume all the ions precipitate ..since the salt is a revisible reaction, why isn;t the Ksp consider at the 1st part?

[Borek]

Do the calculations as you are told and check, how much Ca²⁺ is left in the solution. You will be able to check whether the initial assumption (of 100% precipitation) was correct.

[Albert]

It's a reversible reaction but the common ion effect completely overlaps the reverse path, dissolution.

[Chung Ling Boys]

do you mean that because both ions is present in excess, which causes the common ion efect dominates the reaction instead of the ussual Ksp way to solve problems like this? But does assuming all ions precipitate anything to do with the common ions effect theory? (sorry maybe i dont get what u mean yet..) anyway, i will post the steps to solve this question which i did it according to my teacher's suggestion. Its abit late over here in Malaysia, and i still have a chemistry paper tomorrow. Thanks for the *delete me*(this is a great site!)

[Albert]

Ca²⁺ + C₂O₄^2-  ->  CaC₂O₄  ->    Ca²⁺ + C₂O₄^2-


The second reaction, dissolution, should take place as well as its counterpart, precipitation, according to the value of Ksp.

In the exercise proposed, anyhow, the quantity of ions at the beginning is so large, that it makes the common ion effect paramount.
To sum up, dissolution, actually, does not occur, because the number of moles produced by dissolution is etremely inferior, compared to those introduced at the beginning of the experiment.

Ca²⁺    +     C₂O₄^2-                CaC₂O₄      C₂O₄^2-
0.0050 mol + 0.0051 mol  ->  0.0050 mol  + 0.0001mol (surplus of C₂O₄^2- )

In case you'd have to calculate solubility, pay attention: you must consider a common ion again.

I hope to have been clearer this time.

[AWK]

Now is time to solve a simple quadratic equation:
x(x+0.0001)=Ksp