Question

Calculate the pH of a buffer solution obtained by dissolving 12.0 g of KH₂PO₄(s) and 9.0 g of Na₂HPO₄(s) in water and then diluting to 1 L.

Answer 1

Henderson-Hasselbalch equation is
    pH = pKa + log ([A-]/[AH])
where AH is the weak acid and A is the conjugate base.

Here,

AH = H₂PO₄- ion and A = HPO₄^-2 ion.

For the 2nd dissociation of H₃PO4, pKa = 7.21

Since the A and AH are dissolved in the same water, the volume units cancel and the mole ratio is all that is needed.

The molecular weight of KH2PO4 is 136 and of Na2HPO4 is 142.

Hence,

pH = pKa + log [ moles of Na2HPO4 / moles of KH2PO4]

      = 7.21 + log[(9 /142)/(12/136)]

     = 7.06

Therefore,

pH = 7.06