Question

Calculate the pH of a buffer solution obtained by dissolving 16.0 g of KH2PO4(s) and 27.0 g of Na2HPO4(s) in water and then diluting to 1.00 L. (pKa=7.21)

Answer 1

Convert grams to mols. mols = grams/molar mass.
Then pH = pKa + log(base)/(acid) and solve for pH.

Answer 2

Calculate the molar mass of KH2PO4 and Na2HPO4. Then, determine the moles of the compounds by grams x 1mol/molar mass = moles.
After finding the moles, use the Henderson-Hasselbach equation to solve for pH.
Your Ka=6.2 x 10^-4 and pKa=7.21.
pH=7.21 + log(base/acid)
Base=Na2HPO4
Acid=KH2PO4