Chemistry buffer solution question

Question

I have a question on how to answer this problem. I am reviewing for a test and would really appreciate the help!

Which of the following gives a buffer solution when equal volumes of the two solutions are mixed:

a.) .1M HF and .1M NaF

b.) .1M HF and .1M NaOH

c.) .2 M HF and .1 M NaOH

d.) .1 M HCL and .2M NaF

I know the answer is A, C, and D from the answers in the back of the book, but I do not understand why. Thanks. I appreciate the input!

Answer 1

[Answer:

You know a buffer is formed using a WEAK acid and its conjugate acid base or a WEAK base and its weak conjugate acid.

HF is a weak acid, and F- is its conjugate base, so it would be successful in making a good buffer.

C works as well because if you confirmed it using calculations, you will see that although NaOH is a STRONG base, because its concentration is weaker than that of HF, its purpose is to stimulate the formation of HF's weak conjugate base, which would be F-.  NaOH is all used up in the process, so a buffer will still form.  Same goes for D.

Only B doesn't work because the concentration and volume used for NaOH would match that of HF.  Because so much NaOH is used, a buffer cannot form.

[Answer: see above]