How do you prepare a buffer solution with a pH of 7.40? (1 Viewer)

[csi]

I have this question to answer:
Describe how you would prepare a H2PO4^-/HPO4^2- buffer solution with a pH of 7.40 from the sodium salts of both the H2PO4^- and HPO4^2- ions.

H2PO4^- (aq) + H2O (l) ⇌ HPO4^2- (aq) + H3O^+ (aq)

Ka = 6.2 x 10^-8

I started off by saying that we can determine the amount of acid and conjugate base required using the Henderson-Hasselbach equation...Not sure how to continue.
Note: this is a 6 mark question of my homework

Much appreciated for any help
Thanks!

 

[dasfas]

pH = pKa + log(A-/HA)

Sub in the values from the question to find the log(A-/HA) and then rearrange some more for A-/HA, so now you know the molar ratios they must be in