4.4 - Acids, Bases and pH

Question 1

What is a Brønsted-Lowry acid?

Answer 1

A proton donor.

Question 2

What is a Brønsted-Lowry base?

Answer 2

A proton acceptor.

Question 3

What is the difference between strong and weak acids?

Answer 3

Strong acids fully dissociate in water.

Weak acids dissociate very slightly in water.

Question 4

What is the formula for the ionic product of water?

Answer 4

K_w = [H⁺][OH^-].

Question 5

What is the value of K_w at 298K?

Answer 5

1.00 x 10^-14 mol ² dm^-6.

Question 6

In pure water, what can the formula of K_w be simplified to?

Answer 6

K_w = [H⁺]².

Question 7

How is pH calculated?

Answer 7

pH = -log₁₀ [H⁺].

Question 8

How can you calculate [H⁺]?

Answer 8

[H⁺] = 10^-pH.

Question 9

What does monoprotic mean?

Answer 9

Each molecule of acid produces one molecule of H⁺ ions.

Question 10

What does K_a represent?

Answer 10

The acid dissociation constant.

Question 11

What is the formula for K_a?

Answer 11

K_a = [H⁺] [A ^-] / [HA]

Question 12

How is pK_a calculated from K_a?

Answer 12

pK_a = -log₁₀K_a

Question 13

How is K_a calculated from pK_a?

Answer 13

K_a = 10^-pKa

Question 14

What is a buffer?

Answer 14

A buffer is a solution that resists changes in pH when small amounts of acid or alkali are added.

Question 15

What do acidic buffers contain?

Answer 15

A mixture of a weak acid with one of its salts.

For example, ethanoic acid and sodium ethanoate.

Question 16

How does an acidic buffer work when an acid is added?

Answer 16

The acid reacts with the salt to neutralise the H⁺. This shifts the equilibrium to the left, reducing the H⁺ concentration.

Question 17

How does an acidic buffer work when a base is added?

Answer 17

The OH^- ions react with the H⁺ ions to form water. This shifts the equilibrium to the right which produces more H⁺.

Question 18

What do basic buffers contain?

Answer 18

A weak base with one of its salts.

For example, ammonia and ammonium chloride.

Question 19

How does a basic buffer work when an acid is added?

Answer 19

The H⁺ ions react with the OH^- ions to produce water. This causes the equilibrium to move to the right which produces more OH^- ions.

Question 20

How does a basic buffer work when a base is added?

Answer 20

The OH^- ions react with the NH₄⁺ ions. This causes the equilibrium to shift to the left which removes OH^- ions from the solution.

Question 21

How does a buffer resist changes in pH when diluted by water?

Answer 21

If a small amount a water is added, the water slightly dissociates. The extra H⁺ and OH^- ions push the equilibrium the same amount in both directions.

Question 22

Name two uses of buffers.

Answer 22

In blood.

In shampoo.

In biological detergents.