How do I determine the pH of 1x10^-9 M HCl. I know the answer is pH 7, but how do you get that?

Question

It seems like it should be easy, but I can't figure it out. Thanks so much, final is tomorrow.

Answer 1

This is a famous trick question, but none the worse for that. Why isn't the answer 9? Because you get10^-7 M H+ from the water. Compared with that, the effect of the added HCl is negligible.

If you want a real brain teaser, try to work out the pH of10^-7 M HCl.