CHEM1102 Worksheet 8 – Answers to Critical Thinking Questions
The worksheets are available in the tutorials and form an integral part of the learning outcomes and experience
for this unit.
Model 1: pH
pH 0.50 1.50 2.50 3.50 4.50 5.50 5.75 [H3O+(aq)] 3.2 × 10‐1 3.2 × 10‐2 3.2 × 10‐3 3.2 × 10‐4 3.2 × 10‐5 3.2 × 10‐6 1.8 × 10‐6 1.
2.
The part of the pH value before the decimal point affects the exponent (i.e. the position of the decimal
point).
3.
The part of the pH value after the decimal point affects the coefficient (i.e. the numerical value).
Model 2: Strong and Weak Acids
1.
The major species present are H3O+(aq), Cl-(aq) and H2O(l). There is essentially no “HCl(aq)”.
2.
The major species present are CH3COOH(aq) and H2O(l). The percentage ionization is very small and
there is very little H3O+(aq), CH3COO-(aq).
3.
CH3COO-(aq) is the dominant species only at high pH.
4.
H3O+(aq) + CH3COO(aq)
CH3COOH(aq) + H2O(l)
Ka =
5.
The major species present are CH3NH2(aq) and H2O(l).
6.
CH3NH3+(aq) is the dominant species only at low pH.
7.
(a)
Aspirin is absorbed in the stomach. In the intestine, it is deprotonated.
(b)
Amphetamine is absorbed in the intestine. In the stomach, it is protonated.
Model 3: Conjugate Pairs
1.
(a)
OH-
(b)
H 3O +
2.
(a)
CH3COO-
(b)
NH3
(c)
CH3NH2
3.
(a)
H3S+
(b)
H 2S
(c)
HS–
Model 4: A Solution Containing a Weak Acid
1.
[CH3COOH(aq)]initial = 2.0 M and [CH3COOH(aq)]equilibrium = 2.0 – x.
The small x approximation corresponds to the approximation:
CH3COOH(aq)] equilibrium = [CH3COOH(aq)] initial
HA
HA
2.
x=
3.
(a) pH = 2.229, % dissociation = 0.295
(c) pH = 2.531, % dissociation = 0.590
(b) pH = 2.380, % dissociation = 0.417
(d) pH = 2.681. % dissociation = 0.834
or
pH = -log10(
Acids and Bases
See below.
percentage dissociation
4.
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0
0.2
0.4
0.6
0.8
1
concentration of acetic acid (M)
The major species present H2O(l) and CH3COOH(aq). The amount of dissociation is very small (< 1%)
and so the amount of CH3COO-(aq) and H3O+(aq) is tiny.
5.
The degree of dissociation increases as the weak acid is diluted, even though the amount of dissociated
acid decreases.
Acids and Bases
CHEM1102
2008-J-10
June 2008
Marks
 The structure of methyl 4-aminobenzoate, (E), is given below.
5
O
(E)
O
b
H2N
a
Give the structure(s) of all organic products formed when compound (E) is treated with the
following reagents. If no reaction occurs, write “NO REACTION”.
cold HCl (1 M)
No hydrolysis of the ester under these conditions. The amine group
will be protonated by the strong acid.
O
O
H3N
hot NaOH (4 M)
Treatment with base will lead to hydrolysis of the ester. In the basic
solution, the carboxylic acid will be deprotonated and the amine will
not be protonated.
O
O
+ CH3OH
H2N
hot HCl (4 M)
Hydrolysis of ester. In acidic conditions, the carboxylic acid will not
be deprotonated and the amine group will be protonated.
O
OH + CH3OH
H3N