[_cheers]

Acid dissociation (Ka = 3.6×10-4) of p-nitrobenzoic acid

For a 5.9×10-9-M solution of p-nitrobenzoic acid, calculate the pH.

This is how I did it, which is wrong:

Ka = x(x+ 1E-7)/ 5.9E-9-x = 1.62E-7 or pH 6.79

then i needed to do the fraction of dissoc:

=Ka/10E-6.79 for a very weak acid which is just wrong...

help plz

[enahs]

So, you have a 6 (10^-9) M solution of nitrobenzoic acid.

I assume this is a solution in water?

What is the pH of pure water? What does the autoionization of water tell you about the concentration of H+ and OH- of water? Now look at those and compare it to your solution.

[_cheers]

Oh sorry yes, i should have said that i was using the dissoc. of water and not the solution because it is so weak. Is that what you meaning?

[enahs]

What is the concentration of [H+] for water?

What is the definition of pH?

[AWK]

For your case the approx. formula for calculation of [H₃O⁺] is:
[H₃O⁺]=SQRT(K_a x c + K_w)

For any case, when [H₃O⁺] is known:
alpha= Ka/([H₃O^₊] + K_a)

[Borek]

http://www.chembuddy.com/?left=pH-calculation&right=pH-strong-acid-base