You are to prepare a KH2PO4 and Na2HPO4 solution with pH=7.31.

Question

What should the concentration of [2(HPO4) 2-] / [H2PO4-]

Answer 1

HA + H2O---> H3O+ + A-

Ka=[H3O+][A-]/[HA]

..........HA I H3O+ I A-

I.........I 0.1 M I 0 I 0 I
C........I-2(0.1*0.025) I (0.1*0.025)I (0.1*0.025) I
E.........I 0.1-2(0.1*0.025) I (0.1*0.025) I (0.1*0.025)

Ka=[(0.1*0.025][(0.1*0.025]/[0.1-2(0.1*0.025]

pka=-log(ka)

****This is just the mole ratio concentration. To determine an exact concentration of each you need to know the concentration that you want. What molarity?

Answer 2

I apologize, I am pasting from a word document. before I upload what I think and I posted the wrong one. Ignore original post.

Answer 3

Use the henderson-hasselbalch equation and solve for the ratio.