To simulate blood conditions, a phosphate buffer system with a pH= 7.4 is desired?

Question

What mass of Na2HPO4 must be added to 2 L of 0.35 M NaH2PO4(aq) to prepare such a buffer?

when i worked this out, i got 19.239 but it's wrong

Answer 1

The Henderson-Hasselbalch equation is:

pH = pKa + log (base/acid), where the acid and base make up conjugate pair.

The Ka of H2PO41- is 6.2 x 10^-8

pKa of H2PO41- is -log (6.2 x 10^-8) = 7.21

7.40 = 7.21 + log ([HPO42-] / 0.35)

0.19 = log ([HPO42-] - log (0.35)

0.19 + log (0.35) = log [HPO42-]

0.19 - 0.46 = -0.27 = log [HPO42-]

[HPO42-] = 10^-0.27 = 0.54 M

M = mol / L

mol = M x L = 0.54 mol / L x 2.0 L = 1.08 mol Na2HPO4

1.08 mol Na2HPO4 x (142 g / 1 mol ) = 153 g Na2HPO4 to be added to 2 L of 0.35 M NaH2PO4