Question

Prepare 50 mL of a 50 mM phosphate buffer at your assigned pH6.4 and pka 7.21. Use the Henderson-Hasselbalch equation to determine the ratios of the monobasic and dibasic species you will be using.

Answer 1

pH of phosphate buffer = pka2 + log(HPO4^-2/H2PO4^-)

   pka2 phosphate buffer = 7.21

total no of mol of mol of buffer = 50*10^-3*50*10^-3 = 0.0025 mol

no of mol of HPO4^-2 = x mol

no of mol of H2PO4^- = 0.0025-x mol

6.4 = 7.21 + log(x/(0.0025-x))

x = 0.000335

no of mol of HPO4^-2 = x = 0.000335 mol

no of mol of H2PO4^- = 0.0025-x

          = 0.0025-0.000335

       = 0.002165   mol

[HPO4^-2]/[H2PO4^-] = 0.000335/0.002165 = 0.155

[HPO4^-2] : [H2PO4^-] , 0.15 : 1