# How do you calculate buffer pH for monoprotic acids?

Apr 30, 2015

You use the Henderson-Hasselbalch equation.

A monoprotic acid has one ionizable hydrogen.

${\text{HA" + "H"_2"O" ⇌ "H"_3"O"^+ + "A}}^{-}$

A solution that contains both $\text{HA}$ and ${\text{A}}^{-}$ is a buffer solution, and its pH is given by the Henderson-Hasselbalch equation.

"pH" = "p"K_"a" + log((["A"^-])/(["HA"]))

EXAMPLE

Calculate the pH of a solution that contains 0.20 mol/L acetic acid and 0.10 mol/L sodium acetate. The ${K}_{\text{a}}$ for acetic acid is 1.74 × 10^-5.

Solution

"pH" = "p"K_"a" + log ((["A"^-])/(["HA"])) = -log(1.74 × 10^-5 ) + log((0.10 cancel("mol/L"))/(0.20 cancel("mol/L"))) = 4.76 – 0.30 = 4.46