pH buffer solutions problem?

Question

You have solutions of .2 M HNO(2) and .2 M KNO(2) (K(a) for HNO(2) = 4.00 x 10^-4) A buffer of pH 3 is needed. What volumes of HNO(2) and KNO(2) are required to make 1 liter of buffered solution?

numbers in ( ) are subscripts.

Show how you did it please

Answer 1

pH = pKa + log[Salt]/[Acid]

or log[Salt]/[Acid] = 3-3.398 = -0.398

or x/1-x = 0.4

or 1.4x = 0.4

or x = 285.7 ml of KNO2 soln and 1000-285.7 = 714.3 ml of HNO2 .