Question

1. The pH of a solution made of 1.0 M acetic acid and 0.1 M potassium acetate is (K_a = 1.8 x 10^-5) to which 0.001 mol of KOH has been added: a) pH = 4.74 b) pH = 4.73 c) pH = 8.92 d) pH = 4.46 e) pH = 11.55 The answer is not 4.74!!

2. The pH of a solution made of 1.0 M acetic acid and 0.1 M potassium acetate is (K_a = 1.8 x 10^-5)​ to which 0.001 mol of HCl has been added: a) pH = 1.64 b) pH = 4.75 c) pH = 3.28 d) pH = 8.90

Answer 1

1. The pH of a solution made of 1.0 M acetic acid and 0.1 M potassium acetate is (K_a = 1.8 x 10^-5) to which 0.001 mol of KOH has been added:

pKa of acid = -log 1.8x10-5  = 4.7447

The buffer equation is

CH3COOH + KOH ---------------------> CH3COOK + H2O

1.0 0 0.1 ---- initial moles

------------ 0.001 ------------ ----------- change

0.999 0 0.101 - ------- equilibrium

The pHof this buffer is given by Hendersen equation as

pH = pKa + log [conjugate base]/[acid]

= 4.7447 + log 0.101/0.999

= 3.749

NO CORRECT OPTION GIVEN

2) The pH of a solution made of 1.0 M acetic acid and 0.1 M potassium acetate is (K_a = 1.8 x 10^-5)​ to which 0.001 mol of HCl has been added: a) pH = 1.64 b) pH = 4.75 c) pH = 3.28 d) pH = 8.90

CH3COOK + HCl ---------------------> CH3COOH + KCl

0.1 0 1 ---- initial moles

------------ 0.001 ------------ ----------- change

0.099 0 1.001 - ------- equilibrium

The pHof this buffer is given by Hendersen equation as

pH = pKa + log [conjugate base]/[acid]

= 4.7447 + log [ 0.099/1.001]

=3.7399

NO correct option.

For both questions the options given are not correct.