What is the pH?

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 0.050 mol of HCl were added?

Answer 1

HCl will react with the NaF to form HF and NaCl, thus decreasing the initial [NaF] and increasing the initial concentration of HF.

moles HCl added = 0.05 mols

moles HF formed = 0.05 mols

Final moles HF = 0.05 mols + 0.300 mols = 0.350 mols

Final moles NaF = 0.200 mols - 0.05 mols = 0.150 mols

Using the Henderson Hasselbalch equation we have:

pH = pKa + log [salt] / [acid] and pKa = -log 6.8x10-4 = 3.17

pH = 3.17 + log (0.150 / 0.350)

pH = 3.17 + (-0.37)

pH = 2.80