What is the Ka?

Question

If the pH of an acid is 2.3 when it has been titrated to 1/2 of the equivalence point, what is its K_a?

Answer 1

Since the acid has a Ka value, this tells us that is must be a weak acid. When titrating a weak acid with a base, a buffer is formed. We can use the Henderson Hasselbalch equation for this problem:

pH = pKa + log [conjugate base] / [acid]

At 1/2 the equivalence point, the [conjugate base] = [acid] and log of that = 0, so pH = pKa

Therefore, pKa of the acid = 2.3

Ka = 1x10^-2.3

Ka = 5.0x10^-3