Question

A sample of a diprotic weak acid (H2A) was titrated with 0.0500M NaOH (a strong base) The initial acid solution had a concentration of 0.0250M and had a volume of 50.0mL. For the acid Ka1=1.0*10-3 and Ka2=1.0*10-6.
a) calculate Ve1 and Ve2
b) Calculate the pH after 40.0mL of NaOH was added
c)Calculate the pH after 40.0mL of NaOH was added

Answer 1

Diprotic acid H2A titration

Initial concentration of acid = 0.025 M

moles of acid = 0.025 M x 50 ml = 1.25 mmol

a) Ve1 = volume of base needed to reach first equivalence point

            = 1.25 mmol/0.05 M

            = 25 ml of 0.05 M NaOH needed

Ve2 = volume of base needed to reach second equivalence point

        = 2 x 1.25 mmol/0.05 M

        = 50 ml of 0.05 M NaOH needed

b) pH after 40 ml NaOH was added

moles of NaOH = 0.05 M x 40 ml = 2 mmol

[HA-] remained = 0.5 mmol/90 ml = 0.005 M

[A^2-] formed = 0.75 mmol/90 ml = 0.0083 M

pH = pKa + log(base/acid)

      = 6 + log(0.0083/0.005)

      = 6.18

c) same as b)