O-LevelChemistryStoichiometry5070/22 · Oct/Nov 2020 · Q810 Marks

This question is about some compounds of sulfur. (a) Dilute sulfuric acid reacts with aqueous sodium hydroxide as shown. H2SO4(aq) + 2NaOH(aq) → Na2SO4(aq) + 2H2O(l)

(a)(i)A student titrates 25.0cm³ of dilute sulfuric acid with sodium hydroxide of concentration 0.0150mol/dm³, using litmus as an indicator. A volume of 24.0cm³ of aqueous sodium hydroxide reacts exactly with the dilute sulfuric acid. Calculate the concentration of the dilute sulfuric acid. concentration of dilute sulfuric acid .......................................... mol/dm³[3]
(a)(ii)Describe how to prepare pure dry crystals of sodium sulfate from aqueous sodium sulfate.[3]
(b)Concentrated sulfuric acid oxidises arsenic to arsenic(III) oxide. Complete the equation for this reaction. ......As + ......H2SO4 → AS4O6 + .....H₂O + 6SO2[1]
(c)Sulfur dichloride, Cl− S − Cl, has a simple molecular structure. Complete the dot-and-cross diagram for a molecule of sulfur dichloride. Include only the outer shell electrons. [Figure showing a skeletal structure for sulfur dichloride with empty circles for Cl, S, Cl][1]
(d)The melting point of sulfur dichloride is −121 °C. The boiling point of sulfur dichloride is 59°C. Deduce the state of sulfur dichloride at room temperature. Give a reason for your answer.[2]

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 2020). The mark or mark type is shown in brackets.

[1] 8(a)(i) mol sodium hydroxide = 0.0150 × 24.0 / 1000 OR 3.60 × 10-4 (1)
[1] 8(a)(i) mol sulfuric acid = 1.80 × 10-4 (1)
[1] 8(a)(i) concentration sulfuric acid = 7.2 × 10–3 / 0.0072 mol / dm³ (1)
[1] 8(a)(ii) evaporate solution until crystallisation point / evaporate until solution is saturated (1)
[1] 8(a)(ii) filter off crystals AND wash with organic solvent / wash with cold water (1)
[1] 8(a)(ii) dry crystals with filter paper (1)
[1] 8(b) 4As + 6H2SO4 → AS4O6 + 6H2O + 6SO2
[1] 8(c) : Cl : S : Cl :
[1] 8(d) liquid (1)
[1] 8(d) room temperature is higher than the melting point and lower than the boiling point / room temperature is between the melting point and boiling point (1)

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