NH4F Salt Solution: Acidic or Neutral?

[temaire]

Homework Statement

A solution of NH_4F would be:
a) acidic
b) neutral
c) basic

The Attempt at a Solution

I think the solution would be acidic because it is a salt made of a weak acid and a weak base. Since the Ka for HF is greater than the Kb for NH3, the Ka for NH4+ is greater than the Kb for F-. Thus, the solution is acidic. Am I right?

 

[epenguin]

I think your reasoning is sort of OK, however I would like to add for you to make this secure:

I don't know if you are up with pH's and pK's yet, but the pH of solution equimolar amounts of weak acid and weak base is half way between the pK_a's of both.

These are about 3.2 and 9.2 so your solution will have pH about 6.25

You are right to say it is acidic if by that you mean anything less than exactly 7. But realize only very slightly acidic. In fact in my book I call that neutral, so I don't know what answer they want, that is the trouble with box-ticking answers, but it is more important you understand than get the 'right answer' IMHO.

Important is can you justify, derive, or look up that rule I gave? (Not quite trivial, but useful result).

Secondly and secondarily you might look into WHY HF is a weak acid. Not quite what you might expect considering HCl, HBr, HI.