Titrating a weak acid

Question

An unknown acid has an [A-]/[HA] ratio of 0.2 at pH 3.49. Use the Henderson-Hasselbalch equation to calculate the pK_a of the unknown acid. Use a list of weak acids to suggest possible identities of the unknown.

 

Answer 1

Hello, Patel,

The Henderson-Hasselbalch(H-H) equation relates pH, pK_a, and [A^-]/[HA] concentrations of weak acids by the following:

A weak acid is in an equilibrium:

HA = H⁺ + A^-

which has an equilibrium constant, K_a = ([H⁺] [A^-])/[HA]

The H-H equation is:

pH = pk_a + log [A^-]/[HA]

We are given pH (3.49) and the [A-]/[HA] ratio (0.2).

3.49 = pK_a + log(0.2)

pK_a = 3.49 - log(0.2)

pK_a = 3.49 - (- 0.699)

pK_a = 4.19

[ K_a is therefore 6.47x10^-5]

Bob