Buffer solutions

Question 1

A buffer solution is?

Answer 1

A buffer solution is a mixture that minimises pH changes on addition of small amounts of acid or base.

Question 2

In basic terms what does a buffer solution mixture consist of?

Answer 2

A weak acid, HA.

It’s conjugate base, A-.

Question 3

Explain how a buffer solution is set up using CH3COOH as the weak acid and CH3COONa as the supply of weak base.

Answer 3

In the CH3COOH/CH3COONa buffer system:
- The weak acid, CH3COOH, dissociates partially:
CH3COOH(aq) H(aq) + CH3COO-(aq)
- the salt dissociates completely, generating the conjugate base, CH3COO-.
CH3COO-Na(aq) CH3COO-(aq) + Na*(aq)

The equilibrium mixture formed contains a high concentration of the undissociated weak acid, CH3COOH, and its conjugate base, CH3COO-. The high concentration of the conjugate base pushes the equilibrium to the left, so the concentration of H*(aq) ions is very small.

Question 4

Explain how a buffer solution works?

Answer 4

The buffer solution minimises the pH changes by using the equilibrium:
HA(aq) H*(aq) + A-(aq)

The overall principle is simple:

• The weak acid, HA, removes added alkali.
• The conjugate base, A-, removes added acid.

Question 5

Explain how a buffer solution acts when there’s is an addition of an acid, H*(aq) into the solution.

Answer 5

• [H*(aq)] is increased.
• The conjugate base, A-(aq) reacts with H*(aq) ions.
• The equilibrium shifts to the left, removing most of the added H*(aq) ions.

Question 6

Explain how a buffer solution acts when there’s an addition of an alkali, OH-(aq) to the solution.

Answer 6

• [OH-(aq)] is increased.
• The small concentration of H*(aq) ions react with the OH-(aq) ions.
• HA dissociates, shifting the equilibrium to the right to restore most of the H*(aq) ions that have reacted.

Question 7

Give the formula for calculating the pH using the Henderson-Hasselbalch relationship.

Answer 7

pH = pKa + log [A-]/[HA]