Buffers

Question 1

A- & HA meaning

Answer 1

A- = conjugate base

HA = weak acid

Question 2

A buffer solution

Answer 2

A mixture which minimises the change in pH when small amounts of acid or base are added

• consists of a WEAK ACID & its CONJUGATE BASE

Question 3

How are buffer solutions made?

Answer 3

Either:

• Mixing a weak acid with the salt of the weak acid
• Partially neutralising the weak acid with an aqueous alkali to give the salt & the EXCESS weak acid

Question 4

Buffer System:

CH3COOH(aq) (reversible sign) CH3COO-(aq) + H+(aq)

When acid, H+(aq) is added

Answer 4

• [H+(aq)] increases
• Equilibrium moves to the LEFT to minimise the change and reduce the [H+(aq)]
• The conjugate base, CH3COO-(aq), reacts with H+(aq)

CH3COO- + H+ ————- CH3COOH

• [H+] decreases

Question 5

Buffer System:

CH3COOH(aq) (reversible sign) CH3COO-(aq) + H+(aq)

When alkali, OH-(aq) is added

Answer 5

• OH- react with H+ to form water:

H+ + OH- ——— H2O(l)

• [H+] decreases
• equilibrium moves to RIGHT to increase [H+]
• Weak acid, CH3COOH, dissociates to restore [H+]:

CH3COOH —— CH3COO- + H+

• [H+] increases

Question 6

pH of buffer solutions

Answer 6

Ka = [H+][A-] / [HA]

[H+] = Ka x [acid] / [salt]
= Ka x[HA] / [A-]

pH = -log10[H+]

If concentrations not give calculate:
- moles (n=v/1000 x c)
- concentration =(nx1000) / total v

Question 7

Choosing the weak acid to use in a buffer

Answer 7

A buffer is most effective at removing an acid or alkali when
[HA] = [A-]:

Ka = [H+][A-] / [HA]

• pH of buffer is the same as the pKa of the weak acid
• pH at which the buffer works is typically +-1pH unit of the pH of the pKa value

Question 8

Calculating pH of a buffer

Ratios

Answer 8

Acid : Conjugate base
[H+] / Ka = [HA] / [A-]

Conjugate base : Acid
Ka / [H+] = [A-] / [HA]