O-LevelChemistryAcids, Bases & Salts5070/22 · Oct/Nov 2014 · Q410 Marks

Sulfuric acid reacts with the alkali sodium hydroxide. H2SO4 + 2NaOH → Na2SO4 + 2H2O

(a)Write the ionic equation for this reaction.[1]
(b)The graph below shows how the pH changes when aqueous sulfuric acid is added slowly to 45.0 cm³ of 0.150mol/dm³ sodium hydroxide until the acid is in excess. [Figure showing a titration curve with pH on y-axis and volume of acid added/cm³ on x-axis]
(b)(i)What volume of acid has been added when the pH is 7?[1]
(b)(ii)Use your answer to part (i) to calculate the concentration, in mol/dm³, of the sulfuric acid. concentration = ................................ mol/dm³[3]
(c)The experiment was repeated using ethanoic acid of the same concentration as the sulfuric acid. The same volume and concentration of aqueous sodium hydroxide was used.
(c)(i)The volume of ethanoic acid required to neutralise the aqueous sodium hydroxide was twice as great compared with the volume of sulfuric acid. Explain why.[1]
(c)(ii)Suggest the value of the pH after excess ethanoic acid has been added.[1]
(d)Sulfuric acid is one of the acids present in acid rain.
(d)(i)Suggest how sulfuric acid is formed in the atmosphere.[2]
(d)(ii)State one effect of acid rain on human health.[1]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2014). The mark or mark type is shown in brackets.

[1] A4 (a) H⁺ + OH⁻ → H2O (1)
[1] A4 (b) (i) 20 (cm³)/0.02 dm³ (1)
[1] A4 (b) (ii) mol KOH = 0.15 × 45/1000 OR 6.75 × 10⁻³/0.00675 (1)
[1] A4 (b) (ii) mol H2SO4 = 0.003375/0.0034 (1)
[1] A4 (b) (ii) concentration = 0.003375 × 1000/20 = 0.17/0.169 (1)
[1] A4 (c) (i) ethanoic acid has 1 mol of ionisable H per mol of acid/H2SO4 has 2 per mol of acid/ethanoic acid is monobasic/H2SO4 is dibasic/ethanoic acid has one acidic hydrogen (ion)/sulfuric acid has 2 acidic H⁺ ions/ethanoic acid has half as much ionisable hydrogen (1)
[1] A4 (c) (ii) any value between 3 and 6.9 inclusive (1)
[1] A4 (d) (i) sulfur dioxide/SO2 (1)
[1] A4 (d) (i) (sulfur dioxide) oxidised further/(sulfur dioxide) reacts further to form sulfur trioxide (1)
[1] A4 (d) (i) oxidation product reacts with water to form sulfuric acid/SO3 reacts with water to form sulfuric acid (1)
[1] A4 (d) (ii) irritates skin/irritates eyes/irritates nose/irritates mouth (1)

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