How many moles of NaOH need to be added to 200.0 mL of a 0.200 M solution of HF to make a buffer with a pH of 4.00? (Ka for HF is 6.8 × 10⁻⁴)

Answer 1

Use the Henderson Hasselbalch equation:

pH = pKa + log [conj.base]/[acid]

pKa = -log Ka = 3.17

4.00 = 3.17 + log [F-]/[HF]

log [F-]/[HF] = 0.833

[F-]/[HF] = 6.81

Looking at an ICE table:

HF + OH- ==> F-

0.04....x............0.....I

-x........-x..........+x....C

0.04-x...0.........x.....E

x/0.04-x = 681

x = 0.035 = moles NaOH