21 - Buffers And Neutralisation

Question 1

What is a buffer solution

Answer 1

System that minimises change in pH when small amounts of acid or base are added

Question 2

What two components are buffer solutions made from

Answer 2

Weak acid (HA)
- removes added alkali 

Conjugate base (A-)
- removes added acid

Question 3

List two ways of preparing a acid buffer solution

Answer 3

Preparing from a weak acid and it’s salt

Préparation by partial neutralisation of the weak acid

Question 4

How to prepare a buffer from

A weak acid and its salt

Answer 4

Mixing weak acid (ethanoic acid) with its salt (sodium ethanoate)

Source of weak acid
Ethanoic acid is added to water - partially dissociates = very little ethanoate ions

Source of conjugate base
Salts of weak acid = ionic compound
When added to water completely dissolves - fully dissociates into CH3COO-

Question 5

How to prepare a buffer from

Partial neutralisation of the weak acid

Answer 5

Adding aqueous solution of alkali = NaOH
To excess weak acid

Weak acid is partially neutralised by alkali forming salt and water
Salt fully dissolves = conjugate base

Weak acid is in excess, so partially dissociates = weak acid

Question 6

What is the main equilibrium equation formed in a buffer solution

Answer 6

CH3COOH CH3COO- + H+

• when more CH3COO- is added, equilibrium is shifted further left
• reducing the concentration of H+ to barely anything

So CH3COOH and CH3COO- act as reservoirs that act independently to remove added acid and alkali
- by shifting equilibrium

Question 7

How do buffers remove added acid

Answer 7

(H+) increases
H+ react with A-
Equilibrium shifts to the left
Removing most H+ ions

Question 8

How do buffers remove added alkali

Answer 8

(OH-) increase
OH- reacts with small concentration of H+ to form water
HA dissociates shifting equilibrium to the right to restore H+ ions

Question 9

When is a buffer most effective at removing added acid and alkali

Answer 9

When there are equal concentrations of HA and A-

Question 10

When HA = A-

Answer 10

The pH of the buffer solution is the same as the pKa of HA

The operating pH is over 2 pH units

Question 11

How to calculate pH of buffer solution

Partial neutralisation

Answer 11

1- calculate amount in mol of NaOH
2- find n(A-) from NaOH
3- calculate amount in mol of HCOOH used
4- calculate amount remaining by taking away NaOH
5- use Ka equation to find H+

Question 12

How to calculate pH of a buffer solution

Weak acid + it’s salt

Answer 12

Calculate amount in mol of HCOOH and HCOO-
Calculate concentrations
Use equation