3.

1: Exercise
Section A
For each question there are four possible answers, A, B, C and D. choose the one you
consider to be correct.
1. Buffers present in blood are __________.
A. carbonate/carbonic acid buffer
B. phosphate buffer
C. plasma proteins buffer
D. all of the above

2. Which of the following do not behave as a buffer?

A. Ammonium chloride and sodium hydroxide.
B. Sodium ethanoate and ethanoic acid.
C. Sodium propanoate and propanoic acid.
D. Ammonia chloride and ammonia.

3. __________ play the role of moderating the pH of different solutions during

reactions.
A. Buffers
B. Acids
C. Conjugate base
D. Organic salts

4. Blood pH in humans is around __________.

A. 7.00
B. 7.20
C. 7.45
D. 7.40
5. A buffer solution contains 0.33 M sodium acetate (CH 3 COONa ) and 0.54 M acetic
acid (CH 3 COOH ), pKa = 4.8. Calculate the pH of this buffer solution.
A. 4.59
B. 5.01
C. 4.37
D. 6.01
6. There are two types of buffer solutions namely, acidic buffers and alkaline buffers.
Which statement describes an acidic buffer solution?
A. A combination of a weak base and the salt of the weak base with a strong
acid.
B. A combination of a strong base and the salt of the strong base with a weak
acid.
C. A combination of a weak acid and the salt of the weak acid with a strong
base.
D. A combination of a strong acid and the salt of the strong acid with a weak
base.
7. A common example of an alkaline buffer is a mixture of __________ and
__________.
A. ammonia and ammonium chloride
B. ethanoic acid and sodium ethanoate
C. hydrochloric acid and sodium hydroxide
D. carbonic acid and sodium carbonate

8. As the pKa value of an acid decreases, the acid will be __________.

A. weaker
B. converted to a basic solution
C. stronger
D. converted to a neutral solution

9. The addition of hydrofluoric acid and __________ to water would produce a buffer
solution.
A. NaCl
B. HCl
C. NaOH
D. NaF
−¿¿
10. What is the Kb value for the nitrite ion, NO 2 .Ka for nitrous acid, HNO2 , is 7.1 ×10-4.
A. 1.4 × 10-11
B. 7.1 × 10-18
C. 4.2 × 10-12
D. 7.1 × 10-4

11. The pH of a 0.02M monobasic acid is 3.6. What is the pKa for the acid?
A. 5.5
B. 2.0
C. 3.2
D. 4.0
12. Given that in a buffer solution, the concentration of H +¿¿ ions is 6 × 10-8 mol/dm3
and Kw = 1 × 10-14 mol2/dm6. Calculate the concentration of OH −¿¿ ions in the
solution and the pOH value respectively.
A. 4.1 × 10-7 and 6.0
B. 1.6 × 10-7 and 7.1
C. 2.5 × 10-8 and 6.8
D. 1.6 × 10-7 and 6.8

13. Blood pH in humans is around 7.4 and a rise of pH above 7.45 leads to the condition
called __________ that causes muscle spasms and respiratory paralysis.
A. acidosis
B. alkalosis
C. hypertension
D. none of the above
14. The expression for the dissociation constant K a for the weak acid HA can be written
as __________.
A. K a = log ¿ ¿ ¿
B. K a = -log ¿ ¿ ¿
[ HA ]
C. K a =
¿¿
D. K a = ¿ ¿ ¿

15. Bongani a student at Entumbane High School was carrying out an experiment on
preparations of buffer solutions.She prepared a buffer by mixing 0.15 mol/dm 3 of
propanoic acid of p K a = 4.9 and 0.08 mol/dm3 of sodium propanoate. Calculate the
resulting pH of the buffer solution.
A. 2.6
B. 5.2
C. 4.6
D. 4.8

16. Bicarbonate buffer system is what keeps the blood pH constant. However, carbonic
acid, H 2 CO3 is unstable and decomposes to form __________ and __________.
A. carbon dioxide and hydrogen
B. bicarbonate ion and carbon dioxide
C. carbon dioxide and water
D. hydrogen carbonate and carbon dioxide

17. A number of minerals in soil help to buffer against changes in pH when an acid or
base is added. At a pH above 7, some mineral oxides help to minimise changes in pH.
Choose the correct pair of some of the mineral oxides which play this role.
A. Calcium and magnesium oxides
B. Sodium and silicon oxides
C. Ammonium and sodium oxide
D. Sulphur and potassium oxides

18. Changes in soil pH can affect plants in a variety of ways, especially by diminishing the
fraction of nutrients in soil that are available to the plants and increasing uptake of
undesirable minerals like __________.
A. potassium
B. magnesium
C. phosphorous
D. aluminium

19. 1 mol dm-3 NaCl and 1 mol dm-3 HCl are present in an aqueous solution. The solution
is __________.
A. a buffer with pH less than 7
B. not a buffer with pH greater than 7
C. a buffer with pH greater than 7
D. not a buffer with pH less than 7
20. The value of the ionic product of water at 120℃ is __________.
 A. less than 1 × 10-14
B. greater than 1 × 10-14
C. equal to 1 × 10-14
D. equal to 1 × 10-7
Explanation: The degree of ionization is directly proportional to temperature. So the
concentration of H+ and OH- ions increases with the rise of temperature and the ionic
product becomes greater than 1 × 10-14.
21. An acidic buffer solution can be prepared by mixing solution of __________.
A. Hydrochloric acid and sodium chloride
B. Sulphuric acid and sodium sulphate
C. Sodium methanoate and methanoic acid
D. Ammonium chloride and ammonia

Section B
1. Buffers are important in maintaining blood pH constant, any pH change without
being controlled can lead to severe conditions. From the list of answers given below,
give the correct answer to the questions.
7.35 7.45 7.40
a. Blood pH in humans is around __________.
Answer: 7.40
b. If blood pH is __________, it leads to a condition that causes depression of
the central nervous system.
Answer: 7.35
c. If blood pH is at __________, it leads to a condition that causes muscle
spasms and respiratory paralysis.
Answer: 7.45
2. From the list below select the most appropriate match that will make a buffer.
CH 3 COONa ,CO 2 , NH 3 , NH 4 Cl , CH 3 CH 2 CH 2 COONa , CH 3 CH 2 COONa , NaOH , HCl ,
2−¿ ¿
HPO 4 , NaNO2,

a. Ethanoic acid CH 3 COONa

b. Ammonia NH 4 Cl
c. Carbonic acid CO 2
d. Butanoic acid CH 3 CH 2 CH 2 COONa
e. Nitrous acid NaNO2
2−¿ ¿
f. Phosphoric acid HPO 4

3. __________ is a combination of a weak acid and the salt of the weak acid, often a Na
salt with a strong base.
Answer: Acidic buffer
4. An acidic buffer solution can be prepared by mixing solution of sodium ethanoate
and __________.
Answer: ethanoic acid
5. A common example of an alkaline buffer is a mixture of ammonium chloride and
__________.
Answer: ammonia
6. Using a mixture of ammonia and ammonium chloride solutions ( NH 3 , NH 4 Cl). Addition of
NH 4 Cl results in the equilibrium shifting to which direction?

Answer: left
7. Carbonic acid, H 2 CO3 is unstable and decomposes to form __________and water.
Answer: CO2
8. The expression for the dissociation constant K a for the weak acid HA can be written
as_____________.
Answer: K a = ¿ ¿ ¿

Section C
1. Butanoic acid, CH 3 CH 2 CH 2 COOH is a weak acid commonly known as butyric acid.The
acid is of considerable commercial importance as a raw material in the manufacture of
esters of lower alcohols for use as flavouring agents, its anhydride is used to make
cellulose butyrate, a useful plastic.
a. What do you understand by an acid being described as a weak acid?[1]
Answer: It only partially dissociates in a solution
b. Write an equation to show the dissociation of butanoic acid,CH 3 CH 2 CH 2 COOH , into
its ions.[2]
Answer: CH 3 CH 2 CH 2 COOH (aq)⇋CH 3 CH 2 CH 2 CO O
−¿(aq)¿
+ H +¿(aq)¿
c. The acid dissociation constant, Ka, of butanoic acid at 25℃ is 1.52 x 10-5 mol/dm3. Write
an equation to show an expression for Ka for butanoic acid.[2]
Answer: K a =¿ ¿ ¿
d. Calculate the pH of 0.045 mol/dm3 butanoic acid at 25℃ . Show your working and
include an expression for pH in your answer.[4]

Answer: ¿= (K a × [ CH 3 CH 2 CH 2 COOH ]) 1/2

¿= (1.52 x 10-5× 0.045)1/2
¿= 8.27 x 10-5 mol/dm3
 pH = -log10¿
pH = -log10 (8.27 x 10-5 mol/dm3)
pH = 3.08
2. The body's chemical buffer system consists of three individual buffers namely;
carbonate/carbonic acid buffer, phosphate buffer and plasma proteins buffer. However,
the third buffer is the most plentiful, the first is usually considered the most important
since it is coupled to the respiratory system.
CO 2 (aq) + H 2 O(l)⇋ HCO−¿(aq)¿
3 + H
+¿(aq) K a ¿
= 7.90 x 10-7 mol/dm3
a. Define the term buffer.[2]
Answer:A buffer solution is a solution which minimise a change in pH upon addition of an
acid or base.
b. Using the equation and K a value above, calculate the pH of normal blood plasma, if the [
−¿ ¿
HCO3 ]/ [CO 2] ratio is 10:0.5.[3]

Answer: K a =¿ ¿
10
7.90 x 10-7 = [ H +¿¿ ]
0.5
[ H +¿¿ ] = 3.90 x 10-8 mol/dm3
pH = -log10 [ H +¿¿ ]
pH = 7.40
c. Explain how the system acts as a buffer when H +¿¿ ions are added to the blood plasma.
[2]
−¿ ¿
Answer: This equilibrium responds by shifting to the left due to the reaction of HCO3 ions
+¿¿ +¿¿
and H ions, thus removing the excess H ions.
3. A buffer solution is made by mixing together solutions containing ammonia and
ammonium chloride. K a for the ammonium ion is 5.62 x 10-10 mol/dm3.

a. Upon addition of small amounts of an acid to this buffer, the pH does not significantly
change. With the aid of equations, give a brief explanation for this result.
Answer: The H +¿¿ ions from the acid added will react with OH −¿¿ ions or NH 3molecules
according to the reactions below, thus removing the excess H +¿¿ ions.

H
+¿(aq)¿
+ OH −¿(aq)¿⇋ H 2 O(l)
NH 3 (aq) + H +¿(aq)¿⇋ NH +¿(aq
4
)¿

b. Upon addition of small amounts of an alkali to the buffer solution, the pH does not
significantly change. With the aid of equations, give a brief explanation for this result.
+¿¿
Answer: TheOH −¿¿ ions from the added alkali will react with NH 4 ions according
to the reaction below, thus removing the excess OH −¿¿ ions.
NH 4
+¿(aq )¿
+ OH −¿(aq)¿⇋ NH 3 (aq) + H 2 O(l)

c. Calculate the pH of a buffer solution at 25℃ containing 0.45 mol/dm3 ammonia and
0.25 mol/dm3 ammonium chloride.
⇋ NH 3 (aq) + H +¿(aq)¿
+¿(aq )¿
Answer: NH 4
K a =[ NH 3 ]¿ ¿
0.45
5.62 x 10-10 = [ H +¿¿ ]
0.25
[ H +¿¿ ] = 3.12 x 10-10 mol/dm3
pH = -log10 [ H +¿¿ ]
pH = 9.51
d. Calculate the pOH of the buffer solution in (c) at 25℃ .
Answer: Kw = [ H +¿¿ ] [OH −¿¿]
1 x 10-14 = 3.12 x 10-10 [OH −¿¿]
[OH −¿¿ ] = 3.21 x 10-5 mol/dm3
−¿¿
pOH = -log10 [OH ]
pOH = 4.49
4. An acidic buffer is a combination of a weak acid and the salt of the weak acid (often a Na
salt) with a strong base. It has a pH less than 7. Give one example of an acidic buffer.
Answer: ethanoic acid and sodium ethanoate
5. An alkaline buffer is a combination of a weak base and the salt of the weak base with a
strong acid. It has a pH greater than 7. Give one exampleof an alkaline buffer.Answer:
ammonia and ammonium chloride
6. Below is a table which shows pKa values for 3 different acids
Acid pKa
C 6 H 5 COOH 9.9
C H 3 COOH 4.8
CF 3 COOH 0.2

a. Calculate the Ka value for

I. C 6 H 5 COOH
Answer: Ka = anti log (-pKa)
Ka for C 6 H 5 COOH = anti log10 (-9.9)
Ka for C 6 H 5 COOH = 1.26 x 10-10 mol/dm3
II. C H 3 COOH
Answer: Ka for C H 3 COOH = anti log10 (-4.8)
Ka for C H 3 COOH = 1.58 x 10-5 mol/dm3
III. CF 3 COOH

Answer: Ka forCF 3 COOH = anti log10 (-0.2)

Ka forCF 3 COOH = 6.31 x 10-1 mol/dm3
b. State which is the strongest acid and which is the weakest acid.
Answer: The strongest acid is CF 3 COOH and the weakest is C 6 H 5 COOH