A solution of dihydrogenphosphatehydrogenphosphate buffer co

A solution of dihydrogenphosphate/hydrogenphosphate buffer contains 1.70 M K₂HPO₄ and has a pH of 8.21. Calculate the molarity of NaH₂PO₄ in the solution.

NaH₂PO₄(aq) = HPO₄^2-(aq) + H⁺(aq)

Solution

pH = pKa + log([A-]/[HA])

pKa = -log10(6.2*10^-8) = 7.21

8.21 = 7.21+ log10([K2HPO4]/[NaH2PO4])

[K2HPO4]/[NaH2PO4] = 10

[NaH2PO4] = 1.7/10 = 0.017M.........ANS