Chemical equilibrium — Question 9

QCAA 2025, Paper 2 · 5 marks

Q9 · 2025 · Paper 2Unclassified5 marks

QUESTION 9 (5 marks)

Yellow chromate ions (CrO₄

⁻) are in equilibrium with orange dichromate ions (Cr₂O₇

²⁻) in an

aqueous solution.

⁻(aq) + 2H⁺(aq) ⇌ Cr₂O₇

²⁻(aq) + H₂O(l)

2CrO₄ Yellow Orange

Two experiments were conducted to investigate the effect of pH on the position of equilibrium.

	$\text{[math]}$ [CrO4−](mol L−1)	$\text{[math]}$ [H+](mol L−1)	$\text{[math]}$ [Cr2O72−](mol L−1)	$\text{[math]}$ Kc at 25 °C
Experiment 1		$\text{[math]}$ 1.0 × 10−4		$\text{[math]}$ 4.2 × 1014
Experiment 2	$\text{[math]}$ 1.2 × 10−3		$\text{[math]}$ 6.0 × 10−2	$\text{[math]}$ 4.2 × 1014

a) Deduce the colour of the solution at equilibrium for experiment 1. Explain your reasoning.

[2 marks]

b) Calculate the pH of the solution at equilibrium for experiment 2. Show your working.

[3 marks]

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9a)	K_c is greater than 1; therefore, the equilibrium lies towards the product, so the solution will be orange.	• explains K_c is greater than 1; therefore, equilibrium lies towards the product [1 mark] • deduces that the solution will be orange [1 mark]
9b)	$\text{[math]}$ Let Hx+=22143 22227c246.0 104.2 10(1.2 1)Cr(OCr0)OHKx−−−−+=××=×	• provides appropriate substitution [1 mark]
	$\mathrm{x}^{2}=1.0\,\times 10^{-10}\,$ $\mathrm{x}=1.0\times 10^{-5}$	$\text{[math]}$ • determines [H+] [1 mark]
	$\mathrm{p}\mathrm{H}=\,-\mathrm{l}\mathrm{o}\mathrm{g}(10^{-5})=5$	• calculates pH is 5 [1 mark]

Chemical equilibrium — Question 9

QUESTION 9 (5 marks)

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