pH value from pka

Question

Calculate the pH for a solution of 0.15 M CH​3C​ OOH, given that the p​K​a​ for the acid is 4.75.

Answer 1

CH3COOH ===> CH3COO^- + H⁺

Ka = [CH3COO^-][H⁺]/[CH3COOH]

Ka = 1x10^-4.75 = 1.78x10^-5

1.78x10^-5 = (x)(x)/0.15 -x and assuming x is small relative to 0.15 M we can neglect it in the denominator

1.78x10^-5 = x²/0.15

x² = 2.67x10^-6

x = 1.63x10^-3 M = [H⁺]

pH = -log [H⁺] = 2.79