If a buffer solution is 0.540 M in a weak acid (𝐾a=7.4×10−6) and 0.220 M in its conjugate base, what is the pH?

Question

If a buffer solution is 0.540 M in a weak acid (𝐾a=7.4×10−6) and 0.220 M in its conjugate base, what is the pH?

Answer 1

We can use the Henderson Hasselbalch equation:

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

pH = ?

pKa = -log Ka = -log 7.4x10^-6 = 5.13

log [conj.base] / [wk. acid] = log 0.220 / 0.540 = -0.43

pH = 5.13 - 0.43

pH = 4.70