[afelton1]

The pH of a 0.010 M solution of a weak acid is 4.0. What is the value for the ionization constant, Ka, for this acid?
        A. 10-2
        B. 10-4
        C. 10-6
        D. 10-8
        E. 10-10
The answer is C, but i worked it out using the equation [H+]=10-pH and did not get the right answer.  I dont understand why this is so hard!! its so simple...please *delete me*!

[Borek]

Let's try from the other side. How do you calculate pH of the solution of a weak acid?

[afelton1]

am i supposed to do the ice method?

[Borek]

Yes, try ICE but don't put numbers into the equation yet, try to to derive formula for [H⁺] as a function of Ka and acid concentration.