Acids, bases and titration — Question 2

2a)

2b)

8 −

is 1. 6 2 10

K × [1 mark]

• determines

a

2c)

Phenol red changes colour when the concentration of the yellow weak acid form (HIn) equals the concentration of the red conjugate base form (In^–). When [HIn] = [In^–], the K_a of phenol red equals [H⁺] and pH equals the pK_a (7.9). However, the colour change of phenol red occurs over a pH range equal to pK_a ±1, because visible colour change can be observed when the [HIn] : [In^–] changes by a ratio of 1:10.

• identifies that colour change occurs when the [HIn] = [In^–] [1 mark]

• explains that when [HIn] = [In^–], the K_a of phenol red equals [H⁺], and therefore pH equals the pK_a [1 mark]

• identifies that the pH range of colour change is pK_a ±1 [1 mark]

• explains that the pH range of colour change occurs either side of pK_a because colour change is detected when the [HIn] : [In^–] changes [1 mark]