[organotard]

How would I prepare a 500 mL of a buffer solution of pH 6.90 using 0.10 M solution of Na₂HPO₄ and NaH₂PO₄?
THe pKa of the H₂PO₄^-1 is 6.80 pH units.

[sjb]

What equations do you know that may help?

[organotard]

pH=pKa + log(A-/HA)

am I on the right track with this:

let x=mL of A- component of 0.10M Na2HPO4
let y=mL of HA component of 0.10 M NaH2PO4

Na₂HPO₄----->2Na+(aq) + HPO^-2₄(aq)

NaH₂PO₄------->Na+(aq) + H2PO^-1₄(aq)

x+y=500mL

[HPO^-2₄] = x mL soln. (0.10 moles HPO^-2₄  /1000 mL) (1000mL/L) (x)(0.10)/500)   
all that divided over 500 mL

am I doing this right?  I am sooooo confused!

[Borek]

pH=pKa + log(A-/HA)

Express it in terms of x and y, you will have two equations in two unknowns.

[organotard]

if I do the same for x and then do it for y

should I end up with pH= +log ([HPO4  -2] / [H2PO4 -])

If this wrong could you help me with the right equation to use?

[Borek]

Express ratio of A^-/HA concentrations using volumes of solutions used.