Question

50mL of 0.1M weak acid is titrated with 0.1 strong acid. Pka is 5.20

What is the pH before the acid is added

pH after adding 10mL of acid

pH after 50 mL is added

pH after 60mL is added

Thank you for the help, I dont mind doing the amth, mainly need help with the equarions, thanks!

Answer 1

Weak acid titration with strong base [titration of acid with acid is not possible]

(a) pH before addition of acid

HA <==> H+ + A-

Ka = 6.31 x 10^-6 = [H+][A-]/[HA] = x^2/0.1

x = [H+] = 7.94 x 10^-4 M

pH = -log[H+] = 3.10

(b) after 10 ml of base is added

molar concentration of acid remaining [HA] = 0.1 M x 40 ml/60 ml = 0.067 M

molar concentration of salt formed = 0.1 x 10/60 = 0.0167 M

pH = pKa + log(base/acid)

      = 5.20 + log(0.0167/0.067)

      = 4.60

(c) after 50 ml of base is added

molar concentration of salt [A-] = 0.1 x 50/100 = 0.05 M

A- + H2O <==> HA + OH- ....Kb = [HA][OH-]/[A-]

Kb = 1 x 10^-14/6.31 x 10^-6 = x^2/0.05

x = [OH-] = 8.90 x 10^-6 M

pOH = -log[OH-] = 5.05

pH = 14 - pOH = 8.95

(d) after 60 ml of base is added

molar concentration of base = 0.1 x 10/110 = 9.10 x x 10^-3 M

pOH = -log(9.10 x 10^-3) = 2.04

pH = 14 - 2.04 = 11.96

Let me know If all is clear