Calculate the Ka for 0.220 M solution of H3AsO4, pH = 1.50

Remember that the formula to calculare KA is the following

Ka = [H + ion]*[conj. base] / [acid]

Now that you have the data you can do as the following:

Ka = [H+]*[base] / [acid]

Ka = [x]*[x] / [acid - x]

Ka = x^2 / [acid - x]

then we go

Ka = [.032]^2 / [.220 -.032]

Ka = 5.4 x 10^-3.

Or what is the same

Ka =.0054