1. Chemistry 1011 TOPIC TEXT REFERENCE Acids and Bases
Masterton and Hurley Chapter 4.2 (Review), 13, 14.1, 15.1 (page 427), 21.2 (page589)
Chemistry 1011 Slot 5

2. Note on Topic Sequence
Section 13.4 in the Text includes some Cations as weak acids
Section 13.5 in the Text includes some Anions as weak bases
Section 13.5 also includes material concerning the relation between Ka and Kb
These topics will all be included when we cover Section 13.6 – “Acid-Base Properties of Salt Solutions”
Chemistry 1011 Slot 5

3. 13.5 Weak Bases and Their Ionization Constants
YOU ARE EXPECTED TO BE ABLE TO:
Write an expression for the ionization constant, Kb, for a weak base
Place a list of weak bases in order of strength given values for Kb or pKb
Calculate Kb, given the pOH of a solution of a weak base
Given Kb, calculate the percent ionization of a weak base
Given Kb, calculate the [OH-], [H+] pOH and pH of a solution of a weak base of known concentration
Chemistry 1011 Slot 5

4. Ionization Constants of Weak Bases
Weak bases produce hydroxide ions by reacting with water molecules
Ammonia, NH3, is a weak base
NH3(aq) + H2O(aq) NH4+(aq) + OH-(aq)
cb ca ca cb
In this process,
the NH3 accepts a proton and is a Bronsted-Lowry base
the H2O donates a proton and is a Bronsted-Lowry acid
NH4+ is the conjugate acid of NH3
OH- is the conjugate base of H2O
Organic amines, R-NH2, are also weak bases
Chemistry 1011 Slot 5

5. Ionization Constants of Weak Bases
NH3(aq) + H2O(aq) NH4+(aq) + OH-(aq)
Kb = [NH4+] x[OH-]
[NH3]
It is possible to determine
Kb, knowing the pOH and molar concentration of a weak base
pOH and PH, knowing the value of Kb and the molar concentration of a weak base
Chemistry 1011 Slot 5

6. Calculating Kb from the pH or pOH Value
Calculate the value of Kb for methylamine, CH3NH2 given that the pH of a 0.10mol/L solution is 11.8
CH3NH2(aq) + H2O(aq) CH3NH3+(aq) + OH-(aq)
Kb = [CH3NH3 +] x[OH-]
[CH3NH2]
If pH = 11.8, then pOH = and [OH-] = 6.3 x 10-3
Kb = (6.3 x 10-3) = (6.3 x 10-3)2 = 4.0 x 10-4
(1.0 x x 10-3) (1.0 x 10-1)
Chemistry 1011 Slot 5

7. Calculating [OH-], pH and pOH from Kb
Calculate the pH of a 5.0 x 10-2 mol/L solution of ammonia, given that Kb = 1.8 x 10-5.
NH3(aq) + H2O(aq) NH4+(aq) + OH-(aq)
Kb = [NH4+] x[OH-] = x 10-5
[NH3]
[original] x
D[ ] x x x
[equilibrium] (5.0 x x) x x
Chemistry 1011 Slot 5

8. Solving for x (Approximate Method)
Assume that x << (5.0 x 10-2)
Then: Ka = 1.8 x = x2
(5.0 x 10-2)
x2 = (1.8 x 10-5)(5.0 x 10-2) = 9.0 x 10-7
x = 9.5 x 10-4 mol/L
pOH = pH = 11.0
(Check that [OH-] is << [NH3] )
9.5 x 10-4 mol/L << 5.0 x 10-2 mol/L
Chemistry 1011 Slot 5