Acid/Base Review KEY
1.
Which of the following could be added to a solution of ethanoic acid to prepare a buffer?
A. Sodium hydroxide – Strong Base
B. Hydrochloric acid
C. Sodium chloride
D. More ethanoic acid
2.
Consider an acid-base indicator solution.
HIn(aq) H
+
(aq) + In
–
(aq) color A color B
What is the effect on this acid-base indicator when sodium hydroxide solution is added to it?
A. Equilibrium shifts to the right and more of color B is seen. – NaOH reacts with H + , removing it – needs to be replaced
B. Equilibrium shifts to the left and more of color B is seen.
C. Equilibrium shifts to the right and more of color A is seen.
D. Equilibrium shifts to the left and more of color A is seen.
3.
Which statement explains why ammonia can act as a Lewis base?
A. Ammonia can donate a lone pair of electrons – draw lewis structure
B. Ammonia can accept a lone pair of electrons.
C. Ammonia can donate a proton.
D. Ammonia can accept a proton.
4.
A student has equal volumes of 1.0 mol dm
–3
sodium hydroxide and ammonia solutions. Which statement about the solutions is correct?
A. Sodium hydroxide has a lower electrical conductivity than ammonia.
B. Sodium hydroxide has a higher hydrogen ion concentration than ammonia.
C. Sodium hydroxide has a higher pH than ammonia. – NaOH 13/14
D. Sodium hydroxide has a higher hydroxide ion concentration than ammonia.
5.
Which salts will produce an acidic solution when dissolved in water?
I. CH
3
COOK
II. NH
4
NO
3
III. Al
2
(SO
4
)
3
A. I and II only
B. I and III only
NH
3
8/12
C. II and III only (strong acids with weak bases go through cation hydrolysis)
D. I, II and III
6.
The pH of a solution changes from pH = 2 to pH = 5. What happens to the concentration of the hydrogen ions during this pH change?
A. It decreases by a factor of 1000
B.
C.
D.
It increases by a factor of 1000
It decreases by a factor of 100
It increases by a factor of 100
7.
Some of the most important processes in chemistry involve acid-base reactions.
(i) Calculate the K a
value of butanoic acid, C
3
H
7
COOH, using Table 21 in the Data Booklet. pKa = 4.83 Ka = 10 -4.83
Ka = 1.48x10
-5
1
(1)

(ii) Based on its K a
value, state and explain whether butaoic acid is a strong or weak acid.
Weak acid dues to small Ka (Ka<<1) due to partial dissociation
(2)
(iii) Determine the hydrogen ion concentration and the pH of a 0.010 mol dm
–3
butanoic acid solution. State one assumption made in your calculation.
R C
3
H
7
COOH ↔
I 0.010
C -x
E 0.010
C
3
H
7
COO
0.0
+x
+ H +
0.0
+x x
𝑲 𝒂
=
[𝑪𝟑𝑯𝟕𝑪𝑶𝑶 − ][𝑯 +
[𝑪𝟑𝑯𝟕𝑪𝑶𝑶𝑯]
]
𝟏. 𝟒𝟖𝒙𝟏𝟎
−𝟓
= 𝒙
𝟐
𝟎.𝟎𝟏𝟎
Assume [C
3
H
7
COOH]I = [C
3
H
7
COOH]eqm x
= 𝟑. 𝟖𝟓𝒙𝟏𝟎
−𝟒 pH = -log(3.85x10
-4 ) = 3.42
Ionization of water is insignificant
298K
(4)
(Total 7 marks)
8.
(i) Define the terms acid and base according to the Brønsted-Lowry theory. Distinguish between a weak base and a strong base. State one example of a weak base.
Acid: donates H +
Base: accepts H +
Weak base partially dissociates and strong completely dissociate
NH
3
(3)
(ii) Weak acids in the environment may cause damage. Identify a weak acid in the environment and outline one of its effects.
Carbonic acid – corrodes marble/acidifies water
Sulfuric Acid – burns plants, kills roots by leaching metals
(2)
(Total 5 marks)
9.
A 0.20 mol dm
–3
ammonia solution is placed in a flask and titrated with a 0.20 mol dm
–3
hydrochloric acid solution.
(i) Explain why the pH of the ammonia solution is less than 12.
NH
3
is a weak base and does not fully dissociate, therefore [OH ] < 0.01 mols dm -3 pH = 14 - pOH
(2)
(ii) Estimate the pH at the equivalence point for the titration of hydrochloric acid with ammonia and explain your reasoning. pH < 7 (5-6) because the base is weak making the neutralization point acidic
(2)
2

(iii) State the equation for the reaction of ammonia with water and write the K b
expression for
NH
3
(aq).
+ H
2
O
↔
NH
4
+ + OH NH
3
𝑲 𝒃
=
[𝑵𝑯𝟒 + ][𝑶𝑯 − ]
[𝑵𝑯𝟑]
(2)
(iv) When half the ammonia has been neutralized (the half-equivalence point), the pH of the solution is 8.25. Deduce the relationship between [NH
3
] and [NH
4
+
] at the half-equivalence point.
[NH
3
] = [NH
4
+ ]
(1)
(v) Determine p K b
and K b
for ammonia based on the pH at the half-equivalence point. pH = 8.25 pOH = 14 – 8.25 = 5.75 pKb = pOH at half-equivalence point pKb = 5.75
Kb = 10 -5.75
= 1.78x10
-6
(3)
(vi) Describe the significance of the half-equivalence point in terms of its effectiveness as a buffer.
Half of acid has been neutralized by base which has been converted into a salt. Highest buffer capacity due to 50/50 (base to salt) buffer
(1)
(Total 11 marks)
10.
The graph below indicates the pH change during the titration of 20.0 cm
3
of 0.100 mol dm
–3
of CH
3
COOH(aq) with 0.100 mol dm
–3
KOH(aq). From the graph, identify the volume of KOH(aq) and the pH at the equivalence point.
Volume of KOH = 20 cm 3 at point of inflection/equivalence point
8.5 (half way between 6-11)
11.
Define an acid in terms of the Lewis theory. Deduce, giving a reason, whether NBr
3
is able to function as a Lewis acid or as a Lewis base.
Acid: electron pair acceptor
N has a lone pair of electrons to donate to acid therefore it is a lewis acid
(Total 2 marks)
3