# Exercise 10 | The Properties of Acids and Bases

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.

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 =  = $\frac{{\mathrm{X}}^{2}}{0.20}$ = 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