Calculate the pH of a buffer solution obtained by dissolving 13.0 g of KH 2 PO 4 ( s ) and 32.0 g of Na 2 HPO 4 ( s ) in water and then diluting to 1.00 L

7.58

Explanation:

Given that

A buffer solution obtained by dissolving 13.0 g of KH₂PO₄ and 26.0 g of Na₂HPO₄ in water and then diluting to 1.00 L.

pKa = 7.21.

To get the PH we are going to use Henderson - Hasselblach equation:

PH = Pka + ㏒ [A/AH]

when the molar mass of Na2HPO4 = 142 g/mol 

and A is the conjugate base HPO4-- ions so,

∴[A] = 32g / 142 g/mol 

       = 0.225 M

and when the molar mass of KH2PO4 = 136 g/mol

and AH is the weak acid H2PO4- ions so,

∴[AH] = 13 g / 136 g/mol 

         = 0.096 M

and when we have the Pka value of H3PO4 = 7.21 

so, by substitution:

∴ PH = 7.21 + ㏒ (0.225 / 0.096)

        = 7.58