Chemistry---BUFFERS?

Question

How many mL of .10 NaOH should a student add to 20mL .10HFor if she wish to prepare a buffer with a pH of 3.4?

Answer 1

pH = pKa + log([F-]/[HF])

The pKa of HF is 3.4 (look it up)

3.4 = 3.4 +  log([F-]/[HF]);

Solving, [F-]/[HF] = 1, therefore, [F-] = [HF], so 10 mL of 0.10 M NaOH should be added to 20 mL 0.10 M HF (enough to neutralize half).