Question

You titrate 50.0 mL of phosphoric acid with 0m2 M OH- and reach a pH of 2.148 after delivery of 11.0 mL. What is initial concentration of phosphoric acid? What is the pH after delivery of 22, 25, 44, 66, & 66.5 mL?

Answer 1

pKa1 = 2.15, pKa2 = 7.09, pKa3 = 12.32

At pH = 2.148 = pKa1, the solution is at the first half equivalence, where [H2PO4^-] = [H3PO4]

Quantity of HO- consumed = 0.2 mol/L x0.011L= 0.0022 mol.

Initial quantity of H3PO4 was

2 x 0.0022 mol / 0.050 mL = 0.088 mol/L

After addition of 25 mL HO-:

Very close to the half equivalence, buffer:   pH= 2.15 (relatively constant)

After addition of 44mL HO-:

First equivalence. The solution contains the ampholyte H2PO4^-

pH = (pKa1 + pKa2)/2 = 4.62

After addition of 66 mL HO-:

the solution is at the second half equivalence, where [H2PO4^-] = [HPO4^2-]

pH = pKa2 = 7.09

After addition of 66.5 mL HO-:

Very close to the second half equivalence, buffer:   pH= 7.1 (relatively constant)