Buffer Solutions

Question 1

What is a buffer solution?

Answer 1

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

Question 2

What can’t a buffer solution do?

Answer 2

It can’t prevent the pH from changing completely, but minimises pH changes at least for as long as some of the buffer solution remains.

Question 3

What is a buffer solution a mixture of?

Answer 3

• A weak acid, HA
• It’s conjugate base, A-

It can be made from a weak acid and a weak base.

Question 4

What happens in a CH3COOH/CH3COONa buffer system?

Answer 4

• Weak acid, CH3COOH, dissociates partially;
  CH3COOH H+ + CH3COO-
• the salt dissociates completely, generating a conjugate base, CH3COO-:
  CH3COONa + CH3COO- -> CH3COO- + Na+

Question 5

What does a high concentration of the conjugate base do to the equilibrium?

Answer 5

It pushes the equilibrium to the left so the concentration of H+ ions is reduced. The pH will increase slightly.

Question 6

What is the equation that represents the equilibrium of a buffer solution?

Answer 6

HA H+ + A-

Question 7

What does HA and A- do?

Answer 7

HA removes added alkali and A- removes added acid.

Question 8

What happens to the equilibrium when you add an acid?

Answer 8

Equilibrium shifts to the left (to HA).

Question 9

What happens to the equilibrium when you add an alkali?

Answer 9

Equilibrium shifts to the right (to A-).

Question 10

What happens to the buffer solution on the addition of an alkali, OH-?

Answer 10

• [OH]-(aq) increases
• the small conc. of H ions reacts with the OH- ion so H+ + OH- -> H2O
• HA dissociates, shifting the equilibrium to the right to restore most of the H ions that have reacted.

Question 11

What happens to the buffer solution on the addition of an acid?

Answer 11

• [H+] increases
• the conjugate base, A-, reacts with H+ ions
• the equilibrium shifts to the left removing most of the added H+ ions

Question 12

What does the pH of a buffer solution depend on?

Answer 12

• the acid dissociation constant (Ka) of a buffer system

- the concentration ration of HA and A-

Question 13

What are the uses of buffer solutions?

Answer 13

• Standardising pH meters
• Buffering biological systems e.g. blood
• Maintaining pHs of shampoos

Question 14

What is the biological use of a buffer solution

Answer 14

In biological systems (saliva, stomach, and blood) it is essential that the pH stays ‘constant’ in order for any processes to work properly. e.g. If the pH of blood varies by 0.5 it can lead to unconsciousness and coma.

Question 15

Many salts dissolve in water to produce ions that are not neutral. Explain why.

Answer 15

This is because the ions formed react with the hydroxide and hydrogen ions formed when water dissociates.