Question

What is the pH of each of the following solutions? 0.35 M hydrochloric acid, 0.35 M acetic acid, 0.035 M acetic acid

Answer 1

a. HCl is a strong acid, so it dissociates completely to (formally speaking) H⁺ and Cl^-.

pH = -log[H⁺] = -log(0.35) = 0.456

b. Ka = 1.8x10^-5
CH₃COOH <--> H⁺ + CH₃COO^-
let X = amount of CH₃COOH that dissociates
Ka = 1.8x10^-5 = X²/(0.35-X)
X will be small compared to 0.35
1.8x10^-5 = X²/0.35
X = 0.0025 = [H⁺]
pH = log[H⁺] = 2.6

c. Ka = 1.8x10^-5
CH₃COOH <--> H⁺ + CH₃COO^-
let X = amount of CH₃COOH that dissociates
Ka = 1.8x10^-5 = X²/(0.035-X)
X will be small compared to 0.035
1.8x10^-5 = X²/0.035
X = 7.93 * 10^-4= [H⁺]
pH = log[H⁺] = 3.1