We want to calculate the pH of a 0.20 M aqueous solution of acetic acid (Ka = 1.8 x 10^-5 M) at 25.0°C

1) Can we use the method of successive approximations?

2) Calculate the pH of this solution.

Question

We want to calculate the pH of a 0.20 M aqueous solution of acetic acid (Ka = 1.8 x 10^-5 M) at 25.0°C

1) Can we use the method of successive approximations?

2) Calculate the pH of this solution.

| General Chemistry 3

Accepted Answer

1) At the equilibrium, [HA] = 0.20 – X M and [H₃O⁺] = [A^-] = X M

Method of successive approximations: X << 0.20 ⇒ 0.20 – X ~ 0.20

Let’s verify this approximation:

Ka = $\frac{X^{2}}{0.20 - X}$ = $\frac{X^{2}}{0.20}$ = 1.8 x 10^-5

⇒ X = 1.9 x 10^-3 << 0.2

Yes, we can use the method of successive approximations and X = 1.9 x 10^-3 M

2) [H₃O⁺] = X = 1.9 x 10^-3 M

pH = -log [H₃O⁺] = 2.7

Exercise 10 | Acids and Bases