Acids, bases and titration — Question 4

QCAA 2025, Paper 2 · 5 marks

Q4 · 2025 · Paper 2Unclassified5 marks

QUESTION 4 (5 marks)

Hypochlorous acid (HOCl) is a weak acid formed by dissolving chlorine (Cl₂) gas in water. Hydrochloric acid (HCl) is also formed in this process.

$\mathrm{Cl}_{2}(\mathrm{g})\,+\mathrm{H}_{2}\mathrm{O}(\mathrm{l})\rightarrow HOCl(\mathrm{aq})\,+\mathrm{HCl}(\mathrm{aq})$

The dissociation constant (K_a) for hypochlorous acid is 2.8 × 10⁻⁸.

a) Describe the dissociation of HOCl(aq) and HCl(aq) using balanced chemical equations.

[2 marks]

b) Calculate the pH of a 2.50 M aqueous solution of HOCl. Show your working.

[3 marks]

4a)	( ) ( ) ( ) + − HOCl aq H aq OCl aq + ⇌ ( ) ( ) ( ) + − HCl aq H aq Cl aq $\rightarrow$ +	• describes that HOCl partially dissociates using reversible arrow [1 mark] • describes that HCl fully dissociates using forward arrow [1 mark]
4b)	$\text{[math]}$ []aHOClHOCl+−=K280OClH2.8 10 = 2.5xx−−+=×=	$•\,\mathrm{i}\mathrm{d}\mathrm{e}\mathrm{n}\mathrm{t}\mathrm{i}\mathrm{f}\mathrm{i}\mathrm{e}\mathrm{s}\,\mathrm{t}\mathrm{h}\mathrm{a}\mathrm{t}\,[\mathrm{H}^{+}]\,=\,[\mathrm{O}\mathrm{C}\mathrm{l}^{-}]\,[1\,\mathrm{m}\mathrm{a}\mathrm{r}\mathrm{k}]\,$
	$\text{[math]}$ 4H = 2.6 10x+−=×	• determines that [H⁺] is 2.6 × 10⁻⁴ [1 mark]
	$\text{[math]}$ pHlog H3.6+= −=	• calculates pH [1 mark]