QUESTION 3 (5 marks)
Alizarin yellow R is an acid–base indicator that is yellow in its non-ionised form (HIn) and red in its conjugate base form (In−).
HIn(aq) + H2O(l) ⇌ H3O+(aq) + In−(aq) Yellow Red
The dissociation constant (Ka) for alizarin yellow R is 7.9 × 10−12.
a) Determine the pKa.
[1 mark]
b) Explain the relationship between the pH range of the colour change of alizarin yellow R and its pKa value.
[3 marks]
c) Identify a type of titration for which alizarin yellow R would be a suitable indicator.
[1 mark]
QCAA guide · typeset solution
QCAA sample response and mark allocation
3a) | Indicators are weak acids, therefore,123a[H O ][In ](alizarin yellow R)7.9 10[HIn]K+−−==×123pHlog[H O ]log[7.9 10]+−= −= −×12aplog[7.9 10]11.1−= −×=K | • determines the is 11.1 [1 mark] |
3b) | When [H+] is 10 times greater than the Ka value of alizarin yellow R the In- form dominates and the indicator is yellow. When [H+] is 10 times less than the Ka value of alizarin yellow R the HIn form dominates, and the indicator is red. As indicators change colour at the pH at which the −, the pH range of the colour change for alizarin yellow R is its pKa ± 1. | − > 10 the solution is yellow and when [In ]:[HIn] − • explains when [HIn]:[In ] > 10 the solution is red [1 mark] −[1 mark] • explains that indicators change colour when [HIn]=[In ] • identifies that the pH range of colour change is pKa ± 1 [1 mark] |
3c) | Alizarin yellow R would be a suitable indicator for a weak acid– strong base titration. | • identifies weak acid–strong base titration [1 mark] |
QCAA sample response and marking criteria reproduced from the official guide.
Compare your working with the guide above.