How do you calculate buffer pH for monoprotic acids?

Question

6582 views around the world

You can reuse this answer
Creative Commons License

Answer 1 · 2015-04-30T21:43:40

You use the Henderson-Hasselbalch equation.

A monoprotic acid has one ionizable hydrogen.

$"HA" + "H"_2"O" ⇌ "H"_3"O"^+ + "A"^-$

A solution that contains both $"HA"$ and $"A"^-$ 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 $K_"a"$ for acetic acid is $1.74 × 10^-5$ .

Solution

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