生化習題(英文)

Solutions for the problems about „Calculation of pH in the case of monoprotic acids and bases”



1. What is the pH of a 0.1 M acetic acid solution?

Acetic acid is a weak acid with Ka = 1.86 × 10–5 and in this case cweak acid >>> Ka, that is the equation to use is:
[H+ ] = a acidweak K ⋅ c = 1.0)1086.1( 5 ×× − = 0.0013638 M
pH = -log[H+ ] = -log(0.0013638) = 2.865

2. What is the pH of a 0.1 M ammonia solution?

 Ammonia is a weak base with Kb = 1.75 × 10–5 and in this case cweak base >>> Kb, that is the equation to use is:

[OH– ] = b baseweak K ⋅ c = 1.0)1075.1( 5 ×× − = 0.0013229 M
pOH = -log[OH- ] = -log(0.0013638) = 2.878
pH = 14.00 – pH = 14.00 – 2.878 = 11.122

3. What is the pH of a 0.1 M sodium acetate solution?

Sodium acetate is a weak base, a conjugate base of acetic acid, so:
Ka × K = KW that is acidacetic a acetate W b K K K = = 5- -14 101.86 10 × = 5.376 × 10-10

In this case cweak base >>> Kb, that is the equation to use is:

[OH– ] = b baseweak K ⋅ c = 1.0)10376.5( 10 ×× − = 7.332 × 10-6 M
pOH = -log[OH- ] = -log(7.332 × 10-6) = 5.135
pH = 14.00 – pH = 14.00 – 2.878 = 8.865