[persephones]

I have my board exam tomorrow and i can't seem to wrap my head around this question:

For ethanoic acid, pKa= 4.76, the pH of a dilute solution with a concentration of 1x10^(-10) is almost:
A. 15
B. 5.2
C. 7.0
D. 7.4

The answer is C and since the acid is very dilute we have to consider the ionization of water, too. I don't get how you can calculate the pH considering that acid dissociation is NOT negligible, can someone please explain what steps to take and how the answer is C? Thank you

[Borek]

You are given the acid concentration. Assuming all acetic acid dissociated - by how much would it lower the pH of pure water? Do you really need the exact solution here?

Speaking of exact solution:
http://www.chembuddy.com/?left=pH-calculation&right=pH-weak-acid-base

[persephones]

I don't need the exact solution, I just don't understand the principle behind it. What exactly happens when a weak acid is diluted and why does it become neutral?

[Borek]

I don't need the exact solution, I just don't understand the principle behind it. What exactly happens when a weak acid is diluted and why does it become neutral?

Please reread my post:

Assuming all acetic acid dissociated - by how much would it lower the pH of pure water?

[Arkcon]

Could a very dilute solution, of a weak acid, ever be alkaline?  One of your options is pH 15, a superbase.  Is that remotely possible?  Assuming water is pH 7.00000, can any amount of the weakest acid possible, ever make the solution even 7.000001?

Are you ready for a board exam, presumably a multiple choice standardized test?  Shouldn't you have excluded impossible options from consideration?  This is a tough problem, if you don't realize that the ionization of water has to be accounted for.  But you did know that.