Question

If equal volumes of the following are combined, would they form buffered solutions? Why or why not?

1) .1M NaC2H3O2 and .1M HF
2) .1M HF and .05M NaF

I'm am a bit confused about how I can figure out if these will form buffers or not, so I'd really appreciate it if someone could explain it to me. Thanks.

Answer 1

For a buffer you want a weak acid and a salt of the weak acid (#2 answer fills the bill) or a weak base and a salt of the weak base (example is NH3 and NH4Cl).
#1 is a weak acid (HF) and a salt of A weak acid (but not THE weak acid). Which Ka would you use? Ka for HF or Ka for HC2H3O2 in the Henderson-Hasselbalch equation. I tried some calculations with the HH equation and it appears that adding 5 mL of 0.1M NaOH or 5 mL of 0.1M HCl to a mixture of 100 mL 0.1M HF and 100 mL 0.1M Ac^- produces almost no buffer help. It appears, if I made no errors, that adding the HCl or NaOH would change the pH by as much as 1.3 pH units.