O-LevelChemistryAcids, Bases & Salts5070/21 · Oct/Nov 2023 · Q711 Marks

Dilute ethanoic acid reacts with aqueous calcium hydroxide. 2CH₃COOH + Ca(OH)₂ → (CH₃COO)₂Ca + 2H₂O

(a)A student reacts 25.0cm³ of 0.0100mol/dm³ aqueous calcium hydroxide with dilute ethanoic acid using thymolphthalein as an indicator. A volume of 12.5cm³ of dilute ethanoic acid reacts exactly with the 0.0100mol/dm³ aqueous calcium hydroxide. Calculate the concentration, in mol/dm³, of the dilute ethanoic acid. concentration of dilute ethanoic acid ... mol/dm³[3]
(b)State the colour of thymolphthalein in aqueous calcium hydroxide.[1]
(c)(i)Ethanoic acid is a weak acid. Define the term weak in the phrase weak acid.[1]
(c)(ii)Write the formula of the ion present in aqueous solutions of acids.[1]
(d)Dilute ethanoic acid reacts with sodium carbonate. 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂ Calculate the volume, measured at r.t.p., of carbon dioxide produced, in cm³, when 3.18g of sodium carbonate reacts with excess dilute ethanoic acid. volume of carbon dioxide ... cm³[2]
(e)(i)Sulfur dioxide contributes to acid rain. State one source of sulfur dioxide in the air.[1]
(e)(ii)Describe one method of reducing the amount of sulfur dioxide getting into the air.[1]
(e)(iii)In the Contact process, sulfur dioxide reacts with oxygen. Complete the symbol equation for this reaction. 2SO₂ + O₂ ⇌[1]

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2023). The mark or mark type is shown in brackets.

[1] (a) mol Ca(OH)₂ = (25.00/1000) × 0.010 OR 2.5 × 10⁻⁴
[1] (a) mol CH₃COOH = 5 × 10⁻⁴
[1] (a) = 0.0400 (mol/dm³)
[1] (b) blue
[1] (c)(i) (acid which is) incompletely dissociated / incompletely ionised (in aqueous solution)
[1] (c)(ii) H⁺
[1] (d) mol Na₂CO₃ = 3.18/106 OR 0.03
[1] (d) volume of CO₂ = 720 cm³
[1] (e)(i) burning fossil fuels (containing sulfur compounds)
[1] (e)(ii) flue-gas desulfurisation / using low-sulfur fuels
[1] (e)(iii) 2SO₃

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