pH of a solution

[reising1]

Advertisement - Members don't see this ad

I don't ever remembering considering the pH of water when finding out the pH of a solution. I would always just take the -log(acid concentration).

But in this EK 1001 Chem question
790. What is the pH of a 10^-8 M HCl solution?
Answer: 6.96

This is because they did -log(10^-8 + 10^-7)

I can't ever remember doing this? Is this always done?

 

[reising1]

As a follow up, it seems that when the concentration is dilute, we must consider the contribution of water. In this case, the molarity was 10^-8 (dilute). But when asked about the pH of a 0.01 M solution of strong acid, for example, the answer is pH = 2, because the contribution from water is minimal in that case.

Anyone else agree? Or am I making stuff up? lol