What is the pH of a 1.0 L buffer made with 0.300 mol of HF (Ka = 6.8 × 10⁻⁴) and 0.200 mol of NaF to which 0.070 mol of HCl were added?

Question

What is the pH of a 1.0 L buffer made with 0.300 mol of HF (Ka = 6.8 × 10⁻⁴) and 0.200 mol of NaF to which 0.070 mol of HCl were added?

Answer 1

The added HCl will react with the F- (from NaF) to produce HF (and NaCl).

F- + H+ ===> HF

0.200....0.070.......0.300.............Initial

-0.070...-0.070.....+0.070...........Change

0.130.......0...........0.370............Equilibrium

Use the Henderson Hasselbalch equation to find the pH;

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

pH = ?

pKa = -log Ka = -log 6.8 × 10⁻⁴ = 3.17

pH = 3.17 + log (0.130/0.370)

pH = 3.17 + log 0.351

pH = 3.17 - 0.45

pH = 2.72