[jtruong]

I am trying to work out the pH of 12.0 M HNO₃.

This is what I did:
Since HNO3 is a strong acid, it ionises completely. This means that [H₃O⁺] is 12 mol L^-1.

pH=-log[H30+]
pH=-log[12]
pH= -1.08

The answers say that pH is 1.08. I am pretty sure pH is always greater than zero (not 100% though). This means that my answer is wrong. What am I doing wrong?

[AWK]

Calcualtions of pH for such a big concentration of strong acid are meaningful. You should use the activity coefficients.
Concerning the question about negative pH - this is possble!

[jtruong]

Thanks for replying.

Never heard of activity coefficients. Could you please expand on that. I am guessing this is not in my course though.

By the way, do my calculations look ok?

[Borek]

http://www.chembuddy.com/?left=pH-calculation&right=ionic-strength-activity-coefficients

Your approach is more or less correct, although nitric acid is the weakest of the strong ones, so at such high concentration you should not only take activities into account, but also fact that it will be not 100% dissociated.

[jtruong]

Thanks for the clarification. I have asked my chem teacher and he said that it was a error in the answers.