Unit 1-Acid/Base Equilibria

Question 1

a) What is the dissociation equation for water?
b) What is the equilibrium expression for the above equilibrium?

Answer 1

b) 2H₂O ⇌ H₃O⁺ + OH^-
c) K_w = [H⁺][OH^{<span>-</span>}]

In data booklet

Question 2

The data booklet states the value of the equilibrium constant/ionic product of water at 24 ºC is 10⁻¹⁴

What must the concentrations of hydrogen and hydroxide be at equilibrium?

Answer 2

K_^w = [H⁺][OH^-]

10^-14 = [H⁺][OH^-]

Both must have a concentration of 10^-7 moll^-1

Question 3

Which equation shows the relationship between pH and the hydrogen ion concentration?

Answer 3

pH = -log₁₀[H⁺]

[H⁺] = 10^-pH

Question 4

What are the Brønsted-Lowry definitions of acids and bases?

Answer 4

An acid is a proton donor

A base is a proton acceptor

Question 5

What is formed when an acid loses a proton?

What is formed when a base gains a proton?

Answer 5

Conjugate base

Conjugate acid

Question 6

What is the difference between strong and weak acids/bases?

Answer 6

Strong acids/bases are completely dissociated into ions in aqueous solutions

Weak acids/bases are partially dissociated into ions in aqueous solutions

Question 7

Give an example of a

(a) strong acid
(b) weak acid
(c) strong base
(d) weak base

Answer 7

(a) Hydrochloric acid, sulfuric acid or nitric acid
(b) Ethanoic acid, carbonic acid and sulfurous acid
(c) Solutions of metal hydroxides
(d) Ammonia and amines

Question 8

Compare equimolar weak and strong acids in terms of:

1. pH values
2. Conductivity
3. Reaction rates
4. Stoichiometry

Answer 8

1. Strong acids have lower pH (more hydrogen ions in solution)
2. Strong acids have higher conductivity (more ions in solution)
3. Strong acids react faster (more hydrogen ions in solution)
4. Both will react with the same number of molecules of base

Question 9

The acid dissociation constant is represented by

K_a =?

Answer 9

Question 10

How can you convert K_a to pK_a and vice versa?

Answer 10

pK_a = -log₁₀K_a

Ka = 10^-pK_a

Question 11

How do you calculate the pH of a weak acid?

Answer 11

pH = 1/2pK_a - 1/2log₁₀c

Question 12

A soluble salt of a strong acid and a strong base dissolves in water to produce what type of solution?

Answer 12

Neutral

Question 13

A soluble salt of a weak acid and a strong base dissolves in water to produce what type of solution?

Answer 13

Alkaline solution

Question 14

A soluble salt of a strong acid and a weak base dissolves in water to produce what type of solution?

Answer 14

Acidic solution

Question 15

Explain why a solution of ammonium chloride is acidic.

Answer 15

The solution will contain NH₄⁺ and Cl^- from the salt and H^{<span>+ </span>}and OH^{<span>-</span>} ions from the water.

Some of the NH₄⁺ and OH^{<span>- </span>}will react to form an equilibrium mixture of ammonium hydroxide.

This will leave an excess of H^{<span>+</span>} ions and therefore the solution will be acidic.

Question 16

Explain why a solution of sodium carbonate is basic.

Answer 16

The solution will contain Na⁺ and CO₃^2- from the salt and H^{<span>+ </span>}and OH^{<span>-</span>} ions from the water.

Some of the H⁺ and CO₃^{2-<span> </span>}will react to form an equilibrium mixture of carbonic acid.

This will leave an excess of OH^- ions and therefore the solution will be basic.

Question 17

What does a buffer solution do?

Answer 17

The pH of a buffer solution remains approximately constant when small amounts of acid, base or water are added.

Question 18

How is an acid buffer made?

Answer 18

A solution of a weak acid and one of its salts made from a strong base.

Question 19

How does an acid buffer work?

Answer 19

The weak acid provides the hydrogen ions when these are removed by the addition of a small amount of base.

If an acid (H⁺) is added the conjugate base will form the weak acid.

Question 20

How is a basic buffer made?

Answer 20

Solution of a weak base and one of its salts.

Question 21

How does a basic buffer work?

Answer 21

The weak base removes excess hydrogen ions,

The salt of this base provides the conjugate acid which combines with the hydroxide ions added.

Question 22

Which equation can be used to calculate the approximate pH of an acid buffer solution?

Answer 22

Question 23

What are indicators?

Answer 23

Weak acids which in aqueous solutions the colour of an acid indicator is distinctly different from that of its conjugate base.

Question 24

The expression for acid indicator dissociation constant K_in is

Answer 24

K_In = [H₃O⁺][In^-]/[HIn]

Question 25

The pH range over which a colour change for an indicator occurs can be estimated by the expression?

Answer 25

pH=pK_In±1

Question 26

What is meant by the equivalence point in a titration?

Answer 26

the equivalence point in a titration experiment is reached when the reaction between the titrant (added from the burette) and the titrate (in the flask) is just complete.

Question 27

Which of these indicators

phenolphthalein-pH range 8.2-10.0

bromothymol blue-pH range 6.0-7.6

methyl orange-pH range 3.0-4.4

would be suitable for

(a) strong acid and strong base titration
(b) strong acid and weak base titration
(c) weak acid and strong base titration

Answer 27

(a) bromothymol blue
(b) methyl orange or bromothymol blue
(c) phenolphthalein

Question 28

Indicators won’t work for a particular pair of acids and bases. Which?

Answer 28

Weak acid/weak base

Question 29

Why does the pH of a buffer solution remain approximately constant when H+ ions are added?

Answer 29

The conjugate base of the weak acid absorbs the H+ ions keeping the pH approximately constant.