A-LevelChemistryEquilibria9701/42 · Oct/Nov 2024 · Q319 Marks

The pH of a saturated solution of calcium hydroxide is 12.35 at 298 K. Show that the concentration of hydroxide ions in a saturated solution of calcium hydroxide is 0.0224 mol dm⁻³ at 298 K.

(a)(i)The pH of a saturated solution of calcium hydroxide is 12.35 at 298 K. Show that the concentration of hydroxide ions in a saturated solution of calcium hydroxide is 0.0224 mol dm⁻³ at 298 K.[2]
(a)(ii)Use data given in (i) to calculate the solubility product, Ksp, of calcium hydroxide at 298 K. Include the units of Ksp in your answer. Ksp = units[3]
(a)(iii)A spatula measure of solid calcium chloride is stirred into a sample of saturated calcium hydroxide solution. All of the calcium chloride dissolves. Describe one other observation that would be made and give an estimated value of the pH of the solution obtained. Explain both your answers. observation pH of solution explanation[3]
(a)(iv)Calcium hydroxide reacts with dilute sulfuric acid to form calcium sulfate. Barium hydroxide behaves in a similar way, forming barium sulfate. Explain why calcium sulfate is more soluble in water than barium sulfate.[3]
(b)(i)Some solid calcium is added to an excess of aqueous ethanoic acid, CH₃COOH, and left until all the calcium has reacted. The resulting mixture, mixture D, contains no undissolved solids. Write an equation for the reaction of calcium with CH₃COOH.[1]
(b)(ii)Use formulae of molecules and ions to identify two conjugate acid-base pairs present in mixture D. Pair 1 should consist of organic species. pair 1: conjugate acid conjugate base pair 2: conjugate acid conjugate base[2]
(b)(iii)Write the expression for the Ka of CH₃COOH. Ka =[1]
(b)(iv)The concentration of calcium ethanoate, (CH₃COO)₂Ca, in mixture D is 0.394 mol dm⁻³. The concentration of CH₃COOH in mixture D is 0.270 mol dm⁻³. The K_a of CH₃COOH is 1.74 × 10⁻⁵mol dm⁻³ at 298K. Calculate the pH of mixture D. pH =[2]
(b)(v)Write two equations to show how mixture D can act as a buffer solution. equation 1 equation 2[2]

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2024). The mark or mark type is shown in brackets.

[1] [H⁺] = 10⁻¹²·³⁵ OR [H⁺] = 4.47 × 10⁻¹³
[1] Kw/4.47 × 10⁻¹³ OR pOH = 1.65; [OH⁻] = 10⁻¹·⁶⁵
[1] Ksp = (0.0112)(0.0224)² OR Ksp = [Ca²⁺][OH⁻]²
[1] answer 5.62 × 10⁻⁶
[1] mol³ dm⁻⁹
[1] white solid / white ppt AND pH between 7.01 and 12.34
[1] common ion effect
[1] hydroxide removed by precipitation OR ppt is Ca(OH)₂
[1] lattice energy and hydration energy greater for CaSO₄ OR lattice energy and hydration energy decrease down group
[1] hydration energy decreases more / is dominant factor
[1] enthalpy of solution is more endothermic for BaSO₄ OR is more endothermic down grp
[1] Ca + 2CH₃COOH → Ca(CH₃COO)₂ + H₂
[1] Pair 1: CH₃COOH and CH₃COO⁻
[1] Pair 2: H₃O⁺ and H₂O OR H₂O and OH⁻
[1] [H⁺][CH₃COO⁻] / [CH₃COOH]
[1] [CH₃COO⁻] = 0.788, [CH₃COOH] = 0.270
[1] [H⁺] = 5.96 × 10⁻⁶; pH = 5.22
[1] CH₃COO⁻ + H⁺ → CH₃COOH
[1] OR Ca(CH₃COO)₂ + 2H⁺ → 2CH₃COOH + Ca²⁺; CH₃COOH + OH⁻ → CH₃COO⁻ + H₂O