21.1 Buffer Solutions

Question 1

What is a buffer solution?

Answer 1

A system that minimised pH changes when small amounts of an acid or base are added

Question 2

What does a buffer solution contain?

Answer 2

A weak acid and its conjugate base

Question 3

What does the weak acid do in a buffer solution?

Answer 3

Removes added alkali.

Question 4

What does the conjugate base do in a buffer solution?

Answer 4

Removes added acid

Question 5

When does the solution lose its buffering ability?

Answer 5

When one of the acids/bases is used up

Question 6

What are the two ways to prepare a buffer solution?

Answer 6

• Preparation by partial neutralisation of the weak acid
• Preparation from weak acid and its salt

Question 7

How do you prepare a buffer solution from a weak acid and its salt?

Answer 7

Mix a solution of a weak acid with a solution of one of its conjugate salts (e.g CH3COOH and CH3COONa)

Question 8

How does a buffer solution made from a weak acid and its conjugate salt work?

Answer 8

• Acid added to water partially dissociates
• Salt dissociates into ions, one of which being the conjugate base

Question 9

How do you prepare a buffer solution by partial neutralisation of the weak acid?

Answer 9

Add an aqueous solution of an alkali to an excess of the weak acid. The salt formed is the conjugate base, and the excess acid is the weak acid.

Question 10

What happens when an acid is added to a buffer solution?

Answer 10

• H+ conc increases
• H+ ions interact with the base
• Equilibrium shifts to left, removing H+ ions

Question 11

What happens when an alkali is added to a buffer solution?

Answer 11

• OH- conc increases
• OH- ions interact with H+ ions
• Equilibrium shifts to right, to restore H+ ions

Question 12

How can you change the pH range of a buffer solution?

Answer 12

Change the weak acid used

Question 13

When is a buffer most effective?

Answer 13

When there are equal concentration of the weak acid and conjugate base

Question 14

What happens when [HA (aq)] = [A- (aq)]?

Answer 14

pH of buffer solution has the same pKa value of HA

Question 15

How do you calculate the pH of a buffer solution?

Answer 15

Use formula [H+ (aq)] = Ka * [HA (aq)]remaining / [A- (aq)]

Question 16

When making a buffer solution, do you need more moles of acid or base?

Answer 16

Acid, because you are making a salt and need some acid remaining