Question

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

Answer 1

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