21.1 - Buffer solutions

Question 1

Define buffer solution

Answer 1

A system that minimises pH changes on addition of small amounts of an acid or a base

Question 2

What are the two methods of preparing weak acid buffer solutions?

Answer 2

1. From a weak acid and its salt
2. The partial neutralisation of a weak acid

Question 3

Describe how buffer solutions are prepared from a weak acid and its salt, give a named example

Answer 3

• When ethanoic acid is added to water, the acid partially dissociates into a small amount of ethanoate ions, ethanoic acid is the first component of the buffer solution
• Sodium ethanoate is an ionic compound which completely dissolves and dissociates into its respective ions when added to water, the sodium ethanoate ions are the second component of the buffer solution, acting as the conjugate base component

Question 4

Describe how buffer solutions are prepared from the partial neutralisation of a weak acid, give a named example

Answer 4

• Adding an aqueous strong alkali such as sodium hydroxide to an excess of ethanoic acid partially neutralises the acid
• Forming a solution consisting of the conjugate base (salt) and the remaining unreacted weak acid

Question 5

Describe the position of the equilibrium in ethanoic acid and its effect

Answer 5

• lies towards ethanoic acid
• addition of CHCOO⁻(aq) ions causes the equilibrium to shift further to the left, leaving a solution of mainly two components

CH3COOH(aq) ⇌ H⁺(aq) + CH3COO⁻(aq)

Question 6

Show the equilibrium between the conjugate acid-base pair in a buffer solution

Answer 6

HA(aq) ⇌ H⁺(aq) + A⁻(aq)

Question 7

Describe the function of the conjugate base on the addition of a small amount of an acid, CH3COOH(aq)

Answer 7

1. The concentration of H⁺(aq) increases
2. The H⁺(aq) ions react with the conjugate base, CH3COO-(aq)
3. The position of the equilibrium shifts to the left, removing most of the added H⁺(aq) ions

Question 8

Describe the function of the conjugate acid on the addition of a small amount of an alkali, CH3COOH(aq)

Answer 8

1. The concentration of OH⁻(aq) increases
2. The small concentration of H⁺(aq) ions reacts with the OH⁻(aq) ions, forming water
3. The conjugate acid, CH3COOH(aq) dissociates, releasing and increasing the concentration of H⁺(aq) ions
4. The position of the equilibrium shifts to the right, restoring most of the H⁺(aq) ions

Question 9

When is a buffer most effective at removing ether added acid or alkali?

Answer 9

When there are equal concentrations of both the weak acid and its conjugate base

Question 10

What are happens when the concentration of the weak acid and the conjugate base are equal?

Answer 10

1. The pH of the buffer solution is the same as the pKa value of the acid
2. The operating pH of the acid is about one unit about and below the pH of the pKa value

Question 11

What is the formula to calculate the pH of a buffer solution?

Answer 11

[H⁺(aq)] = Ka x [HA(aq)]
————-
[A⁻(aq)]
-> pH = -log([H⁺(aq)])