Question 19

[McatCrusher]

for this question and similar questions like this, i have always trusted in Ksp and most of the time i got the right anwer. The solution, however, says that i should not do so. but the suggested method is really time taking. I was wondering what you would recommend in tackling questions like this.

[Dr. Ferdinand]

Hello. I'm not sure what you meant by "trusted in Ksp" but there are 2 ways to ask questions about Ksp for the MCAT: (1) where the answer depends on Ksp or (2) where the answer depends on an understanding of solubility and its relation to Ksp as in this particular problem.

Clearly, you need to understand both and the latter just adds one extra step which is described in the explanation. But, it's not time consuming. When you look at the explanation, you'll notice that one third is about theory then the rest is showing 2 different ways to solve the problem.

[admin]

You definitely look at the Ksp. 9/10 questions on the MCAT will only depend on you understanding the rules, as described, around Ksp. However, when you have multiple ions in the same container (as in this question) then you can have a common ion effect and then you should at least take a peak at solubility to determine whether it has a significant effect as compared to the other salts/ksp's in solution.

If you want to read more: www.chem.purdue.edu/gchelp/howtosolveit/Equilibrium/Solubility_Products.htm