Lesson 11: Weak Bases
Q: What are two defining characteristics of a weak base dissociation equation?
A: - They form equilibrium systems
- They produce OHTherefore the general dissociation equation for any weak base is:
Weak Base + H2O (l) ========= Con. Acid + OH- (aq)
Q: How can I identify a weak base?
A: Weak bases are generally ANIONS that have WEAK conjugate acids.
(refer to: How do I properly identify molecules in the acid/base unit?)
MEMORIZE: NH3 is a weak base! (it’s also listed in the weak base column pg.6 data booklet)
Q: Can anions from a salt act as a base?
A: YES.
e.g., What is the [OH-] of a 0.10 M solution of NaCN?
Q: Does the anion have a weak conjugate acid?
A: Yes (HCN). Therefore CN- IS A WEAK BASE.
Step #1: Write a weak base dissociation equation.
CN-(aq) + H2O(l) ====== HCN(aq) + OH-(aq)
Step #2: ICE
I
C
E
0.10M
-X
0.10 –X
0.0M
+X
X
0.0M
+X
X
Step #3: Determine the value of Kb using pg.6
Kb = Kw
=
Ka of the con acid
𝟏.𝟎 𝒙 𝟏𝟎−𝟏𝟒
= 2.0408 x 10-5
−𝟏𝟎
𝟒.𝟗 𝒙 𝟏𝟎
(i.e., HCN)
Step #4: Solve for x from ICE (this represents the [OH-])
[𝑯𝑪𝑵][𝑶𝑯−]
Kb =
[𝑪𝑵−]
√𝟐. 𝟎𝟒𝟎𝟖 𝐱 𝟏𝟎−𝟔 = √𝑿𝟐
2.0408 x 10
-5
=
𝑿𝟐
(𝟎.𝟏𝟎𝑴)
1.42857 x 10-3 M = X = [OH-]
Step #5: Use your answer from #4 to perform whatever roadmap calculations you need.
[OH-] = 1.4 x 10-3 M
2. Calculate the pH of a 1.0 M solution of NaCl.
Q: Is the anion from a weak conjugate acid?
A: No. Therefore Cl- is NOT a weak base. Since there is no reaction,
the pH = 7.0.
3. Calculate the pH of a 0.10 M solution of NaCH3COO.
CH3COO- has a weak conjugate acid (CH3COOH), therefore it’s a
weak base.
CH3COO-(aq) + H2O(l) ===== CH3COOH(aq) + OH-(aq)
I
C
E
0.10M
-X
0.10 –X
Kb = Kw
0.0M
+X
X
=
𝟏.𝟎 𝒙 𝟏𝟎−𝟏𝟒
Ka of the con acid
𝟏.𝟖 𝒙 𝟏𝟎−𝟓
0.0M
+X
X
= 5.55555 x 10-10
(i.e., CH3COOH)
[𝑪𝑯𝟑𝑪𝑶𝑶𝑯][𝑶𝑯−]
Kb =
[𝑪𝑯𝟑𝑪𝑶𝑶−]
√𝟓. 𝟓𝟓𝟓𝟓𝟓 𝐱 𝟏𝟎−𝟏𝟏 = √𝑿𝟐
pOH = -log[OH-]
pOH = -log(7.453 x 10-6 M)
pOH = 5.1276
pKw = pH + pOH
14.000 = pH + 5.1276
pH = 8.87
-10
5.55555 x 10 =
𝑿𝟐
(𝟎.𝟏𝟎𝑴)
7.453 x 10-6 M = X = [OH-]