A question on pH, finding the pH..

Question

100ml hydrochloric acid of pH 4.0 is mixed with 100ml of hydrochloric acid of pH 5.0. what is the pH of the solutionb formed?

i tried to find no. of mols HCl in each of the 2 solutions then add together to find total no of mols then used c=n/v to find the pH of the 0.2L solution but i got the wrong answer. Can someone please lend a hand and tell me what i done wrong?

^^

Answer 1

You followed the right approach.  Not sure why you didn't get the right answer, but here is what I got:

In the pH = 4.0 solution:

[H+] = 1.0 x 10^-4 M.

In 0.100 L, n = 1.0 x 10^-5 mol H+

In the pH = 5.0 solution:

[H+] = 1.0 x 10^-5 M.

In 0.100 L, n = 1.0 x 10^-6 mol H+

In the combined solution:

n = 1.1 x 10^-5 mol H+

c = 1.1 x 10^-5 mol H+ / 0.200 L = 5.5 x 10^-5 M

pH = -log(5.5 x 10^-5 M) = 4.26