organic acids and bases

Question

Enough of a monoprotic weak acid is dissolved in water to produce a 0.0158 M solution. The pH of the resulting solution is 2.59. Calculate the p𝐾a for the acid.

Answer 1

Let the monoprotic weak acid be HA

HA <==> H⁺ + A^-

Ka = [H⁺][A^-] / [HA]

From pH, we can find [H⁺] and [A^-]

pH = 2.59 = -log [H⁺]

[H⁺] = 1x10^-2.59 = 2.57x10^-3 M = [A^-]

Ka = [2.57x10^-3 ][2.57x10^-3 ] / 0.0158

Ka = 4.18x10^-4

pKa = -log Ka = -log 4.18x10^-4

pKa = 3.38