Exercise 10 | The Properties of Acids and Bases
General Chemistry 3 - Exercise 10
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 = = = 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