Chapter 21: Buffers and Neutralisation (21.1-21.3)

Question 1

what are buffer solutions used for?

Answer 1

to minimise changes in pH to a solution on addition of small amounts of an acid or base

Question 2

how does an acid buffer work

Answer 2

Acid buffer solutions contain two components, a weak acid HA and its conjugate base A-:
HA (aq) < – > H+ (aq) + A- (aq)

When an acid (H+) is added, it reacts with the conjugate base (A-) to form HA and the equilibrium shifts to the left.
When alkali (OH-) is added, it reacts with the H+ to form H2O and the equilibrium shifts towards the right.p

Question 3

preparing a buffer solution from a weak acid and a salt

Answer 3

a buffer solution can be prepared by mixing together a solution of a weak acid (eg CH3COOH) and a solution of a salt of that weak acid (eg CH3COONa).
Work out concentrations of acid and salt, assume the concentration of the salt is the same as the concentration of the conjugate base ( [A-] )

Question 4

preparing a buffer solution by partial neutralisation of a weak acid

Answer 4

a buffer solution can be prepared by mixing together an excess of a weak acid (eg CH3COOH) and a solution of an alkali (NaOH)

Question 5

Ka value for buffer solution

Answer 5

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

Question 6

buffer system used in the blood

Answer 6

carbonic acid-hydrogencarbonate

H2CO3(aq) < – > H+(aq) + HCO3-(aq)

Question 7

pH of blood plasma

Answer 7

7.35 - 7.45

ideal = 7.40

Question 8

explain the shape of a pH titration curve

pH on y-axis, volume of base/acid added on x-axis

Answer 8

Initial shallow curve: pH changes slowly as there is still an excess of the original acid/base.

Vertical section: pH increases rapidly on addition of a very small amount of base/acid, acid and base concentrations are similar.

Equivalence point: the centre of the vertical section aka the point of neutralisation

Final shallow curve: pH changes slowly as it is now the base/acid that was added that is now in excess

Question 9

what is an indicator

Answer 9

An acid-base indicator is a weak acid, HA, that has a different colour from its conjugate base, A-.
The colour changes as a response to the shift in equillibrium

Question 10

at the end point of a titration

Answer 10

• There are equal concentrations of HA and A-

- The colour of the indicator is between the two extreme colours

Question 11

how to chose an indicator

Answer 11

you must use an indicator with an endpoint that coincides with the vertical section of the pH titration curve.
The vertical section must go completely through the range which the indicator would change colour.

Question 12

What is the pH of a buffer where HA and A- have the same concentrations?

Answer 12

[H+] = Ka x [HA]/[A-] therefore [HA] and [A-] cancel so pH is equivalent to the pKa value and [H+] is equivalent to Ka

Question 13

How do you work out H+ in a buffer?

Answer 13

Ka x [HA]/[A-]