One of my past exam papers asks the following:

*A fraction of dissociated molecules of a weak monoprotic acid in its 0.09 mol/dm*^{3} aqueous solution is only 10%. What is the Ka?

I used the formula Ka = [H+][A-]/[HA], plotted in and got the answer:

(0.09*0.1)(0.09*0.1)/(0.09) = 9x10^{-4}

But the answer section says Ka = 10^{-3}

Is my calculation wrong?