Question

using the following salts listed below, calculate the moles of acid and conjugate base needed to make 100 ml of 0.50 M, ph 6.5,6.7, and 6.8 phosphate buffer solution. (For H2PO4-, pka=6.64)

Salt Molar mass, g/mol
KH2PO4 136.09
NaH2PO4*H2O 137.99
Na2HPO4*7H2O 268.09
K2HPO4 174.2

Answer 1

pH = pKa + log(base)/(acid)
You want 100 mL x 0.5M = 0.05 mols

6.5 = 6.64 + log (B/A)
Solve for B/A, then
A + B = 0.05.
Two equations and two unknowns, Solve for acid and base mols, then convert to grams from g = mols x molar mass.

I would check myself upon finishing each by substituting grams of each salt back into the HH equation and see if it givs the desired pH.