Question 19

[tjk447963]

On questions like these, do you always have to solve the expression for "s" (solubility)? I answered it by just comparing the Ksp value of all the salts.

[mcat_premed3832]

Comparing solubility (s) and solubility product (Ksp) is similar to comparing [H+] and pH. The concepts are related but definitely not the same. If you use Ksp instead of solubility when the question is asking for solubility, then luck would be the only way to get the answer correct. It will become easier with pattern recognition.

[shaidar1472]

But on this question, the passage clearly indicates in the second paragraph that "the smaller the Ksp value for a salt, the earlier it will precipitate out of the solution relative to other salts." Therefore, wouldn't that be enough information to pick answer choice A, simply because the Ksp is smaller than the rest? The question itself does not ask specifically about the solubility of the salt after all...so why would we have to solve for it?

[mcat_premed3832]

You are correct. In fact, when you look at how students performed in this question (checking the difficulty icon), in general students found it to be an easy question exactly for the reason you stated. Likely no one did any calculations since they are not necessary. The rule of solubility was described in the passage (which is the correct way to answer the question) and then the Explanation went into some theory which can be useful if it was required to calculate or determine ionic solubility (which is not in the case in this problem).

In other words (!!!), in response to the first question in this thread, I should have just said "no."