Question

Calculate the pH for each of the following cases in the titration of 50.0 mL of 0.100 M HClO(aq) with 0.100 M KOH(aq). The ionization constant for HClO can be found here.

(a) before addition of any KOH (b) after addition of 25.0 mL of KOH (c) after addition of 30.0 mL of KOH (d) after addition of 50.0 mL of KOH (e) after addition of 60.0 mL of KOH

Answer 1

pKa = -logKa = = -log(4.0 x10^ -8.) = 7.4

millimoles of HClO= 50 x0.1= 5

(a) before addition any KOH added

pH = 1/2 (pKa - log C)

   = 1/2 (7.4 -log (0.1) ) = 4.2

pH= 4.2

(b) after addition of 25.0 mL of KOH

millimoles of KOH = 0.1 x 25 = 2.5

HClO + KOH ------------------------------> KClO + H2O

5            2.5                                               0               0 -----------------------initial

2.5          0                                                2.5           2.5 -------------------equilibrium

in the solution acid and salt remained so it can form buffer

For acidic buffer

Henderson-Hasselbalch equation

pH = pKa + log[salt/acid]

    = 7.4 + log (2.5/2.5)

pH = 7.4

(c) after addition of 30.0 mL of KOH

millimoles of KOH = 0.1 x 30 = 3.0

HClO + KOH ------------------------------> KClO + H2O

5            3.0                                               0               0 -----------------------initial

2.0          0                                                 3.0          3.0 -------------------equilibrium

in the solution acid and salt remained so it can form buffer

For acidic buffer

Henderson-Hasselbalch equation

pH = pKa + log[salt/acid]

    = 7.4 + log (3.0/2.0)

    = 7.58

pH = 7.58

(d) after addition of 50.0 mL of KOH

millimoles of KOH = 0.1 x 50 = 5

HClO + KOH ------------------------------> KClO + H2O

5            5                                                       0            0 -----------------------initial

0          0                                                         5           5-------------------equilibrium

in the solution salt remained so we have to use salt hydrolysis.

it is the salt of strong base and weak acid so pH should be more than 7

[salt] = salt millimoles /total volume in ml

           = 5/(50+50)

           = 0.05 M

pH = 7 + 1/2[Pka + logC]

   = 7 + 1/2 [7.4 + log (0.05)]

    = 10.05

pH = 10.05

e) after addition of 60.0 mL of KOH

millimoles of KOH = 0.1 x 60 = 6

HClO + KOH ------------------------------> KClO + H2O

5            6                                                 0               0 -----------------------initial

0          1                                                   5              5-------------------equilibirum

in the solution strong base remained

[base ] = 1/total volume = 1/110 = 0.009

pOH = -log[OH-] = -log(0.009) =2.04

pH + pOH = 14

pH = 11.96