Buffers

Question 1

Buffer solution

Answer 1

A system that minimises pH changes when small amounts of an acid or base are added. They contain 2 components: a weak acid and a conjugate base

Question 2

Preparing weak acid buffer solutions

Answer 2

• from a weak acid and it’s salt
• by partial neutralisation of the weak acid
• 2 reservoirs to remove added acid and alkali

Question 3

Conjugate base removed added acid

Answer 3

1) [H+] increases
2) H+ ions react with conjugate base
3) equilibrium shifts left (removes H+ ions)

Question 4

Weak acid removes added alkali

Answer 4

1) [OH-] increases
2) small concentration of H+ reacts with OH- ions

H+(aq) + OH-(aq) -> H2O(l)

3) HA dissociates = equilibrium shifts right to restore H+ ions

Question 5

Most effective buffers

Answer 5

When [HA(aq)] = [A-(aq)]

• pH of buffer solution is the same as the pKa value of HA
• operating pH is over about 2pH units, centred at the pH of the pKa value

Question 6

[H+] calculation for buffers

Answer 6

[H+(aq)] = Ka x [HA(aq)]/ [A-(aq)]

Question 7

Equivalence point

Answer 7

The volume of one solution that exactly reacts with the volume of the other solution. The solutions have reacted exactly and the amounts used match the Stoichiometry of the reaction

Question 8

End point

Answer 8

Indicator contains equal concentrations of acid and conjugate base = colour shows inbetween the 2 extremes

Question 9

Indicator

Answer 9

Weak acid- equilibrium shifts towards the weak acid in acidic conditions and towards the conjugate base is basic conditions

Question 10

On addition of a basic solution containing OH- ions to an indicator

Answer 10

-OH- reacts with H+ in indicator

H+ + OH- -> H2O

• weak acid dissociates = equilibrium shifts right
• colour change to end point colour then conjugate base colour

Question 11

On addition of an acid to an indicator

Answer 11

• H+ reacts with conjugate base
• equilibrium shifts left
• colour change to end point colour and then acid colour

Question 12

What indicator is suitable for a weak acid-weak base titration

Answer 12

No indicator

Question 13

Strong acid/strong base titration

Answer 13

• starts at pH0
• ends at pH 14
• steep in the middle
• pH at equivalence is very close to pH 7

Question 14

Strong acid/weak base titration

Answer 14

• starts at pH 0
• ends at pH 9-10
• slight incline at the end (when buffer is set up)
• pH at equivalence is acidic

Question 15

Weak acid/ weak base titration

Answer 15

• starts at pH 3-4
• ends at pH 9-10
• steep in the middle
• pH at equivalence point is close to pH 7

Question 16

Weak acid/ strong base titration

Answer 16

• starts at pH 3-4
• ends at pH 14
• slight incline at the start then flattening out
• steep in the middle
• pH at equivalence is alkaline