Calculate the ph of a buffer solution obtained by dissolving 13.0 g of kh2po4 (s) and 26.0 g of na2hpo4 (s) in water and then diluting to 1.00 l.

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] = 26g / 142 g/mol

= 0.183 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 + ㏒ (0.183 / 0.096)

= 7.49