Question

A certain weak acid (pK_a = 5.2) is used as a food preservative. Determine the pH of a 0.54 M solution.

Answer 1

Let the weak acid be HA

HA $\to$ H⁺ + A^-

at t =0 0.54_________-_____-

at t = t_eq 0.54-x _____x___x

K_a = $\frac{[H^+][A^-]}{[HA]}$

10^-5.2 = $\frac{[x][x]}{[0.54-x]}$

0.54-x $\mapsto$ 0.54 since x is very less

0.54×10^-5.2 = x²

10^-0.26 × 10^-5.2 = x²

10^-5.46 = x²

1.86 × 10^-3 = x

[H⁺] = 1.86 ×10^-3

[H⁺] = 10^-2.73

pH = 2.73