What Volumes of HNO2 and KNO2 Create a pH 3 Buffer?

[Hemolymph]

Homework Statement

. You have solutions of 0.200 M HNO2 and 0.200 M KNO2 (Ka for HNO2 = 4.00 × 10–4). A buffer of pH 3.000 is needed. What volumes of HNO2 and KNO2 are required to make 1 liter of buffered solution?
A) 500 mL of each
B) 286 mL HNO2; 714 mL KNO2
C) 413 mL HNO2; 587 mL KNO2
D) 714 mL HNO2; 286 mL KNO2
E) 587 mL HNO2; 413 mL KNO2

Homework Equations

pH=pKa+log([A]-/[HA])

The Attempt at a Solution

3=3.4+log([A-]/[HA])
-.4=log([A-]/[HA])
10^-.4
.398=[A-]/[HA]
I get confused conceptually of what to do after this. I know I need .398M of conj base for every 1M of acid

 

[Borek]

.398=[A-]/[HA]

This is one equation. The other is

V_{A^-} + V_{HA} = 1L

You need to combine volume with concentration - and you will have two equations in two unknowns. Just solve.