Question

A solution contains 0.0249 M HBr and 0.270 M acetic acid. What is the pH?

Answer 1

1. Determine [H+] in HBr (strong acid).

[H+] in HBr = 0.0249 M

2. Determine [H+] in acetic acid (weak acid) using ICE (initial, change, equilibrium) table.

----HCOOH = H+ + HCOO-
I----0.270
C----(-x)---------x--------x
E--(0.270-x)---x--------x

Ka = [H+][HCOO-]/[HCOOH] = 1.8 x 10^-5
(x)*(x)/(0.270-x) = 1.8 x 10^-5

Since [HCOOH]/Ka > 100, we can simplify the above equation to:

x^2/0.270 = 1.8 x 10^-5
x =2.204 x 10^-3 = [H+]

3. Determine total [H+].

[H+] = 0.0249 M + (2.204 x 10^-3 M) = 0.02710 M

4. Determine pH.

pH = -log[H+]
pH = -log(0.02710 M) = 1.567