Question

What will happen if a small amount of hydrochloric acid is added to a 0.1 M solution of HF?

A) The percent ionization of HF will increase.
B) The percent ionization of HF will decrease.
C) The percent ionization of HF will remain unchanged.
D) Ka for HF will increase.
E) Ka for HF will decrease

I think A is the correct answer because more H+ ions are being added

Answer 1

Nope. That isn't it. You're right, of course, that H^+ will increase because you are add so many H^+ with the HCl. BUT, this H+ you are adding is a "common ion" to the H^+ in HF. Remember HF is a weak acid.
HF ==> H^+ + F^-
Ka = (H^+)(F^-)/(HF) = about 7E-4

So when you add H^+ to this (the common ion), Le Chatelier's principle says the reaction will shift so as to undo what we've done to it. Since we've increased the H+ by adding HCl, the HF reaction will shift to the left to decrease it's contribution of the H^+ which makes b the correct answer. HF gets slightly larger; H^+ (from HF) gets slightly smaller.