O-LevelChemistryAcids, Bases & Salts5070/21 · May/June 2016 · Q210 Marks

Hydrogen sulfide, H2S, has a simple molecular structure. It is soluble in water.

(a)Suggest one other physical property of hydrogen sulfide.[1]
(b)(i)Write an equation to show the dissociation of hydrogen sulfide.[1]
(b)(ii)Why is aqueous hydrogen sulfide described as a weak acid?[1]
(c)Aqueous hydrogen sulfide reacts with aqueous potassium hydroxide. H2S(aq) + 2KOH(aq) → K2S(aq) + 2H2O(l) What is the minimum volume, in cm³, of 0.150mol/dm³ KOH required to completely react with a solution containing 0.170 g of H2S?[3]
(d)(i)Magnesium reacts with sulfur to make the ionic compound magnesium sulfide. Predict two physical properties of magnesium sulfide.[2]
(d)(ii)Explain, in terms of electrons, how a magnesium atom reacts with a sulfur atom to make a magnesium ion and a sulfide ion.[2]

Mark scheme

Marking points from the official mark scheme (May/June 2016). The mark or mark type is shown in brackets.

[1] 1 mark for any one of: • Low melting point/low boiling point • Does not conduct electricity • Does not conduct heat
[1] H2S → H⁺ + HS⁻ / H2S → 2H⁺ + S²⁻ (1)
[1] Incomplete dissociation/partial dissociation (1)
[3] Moles of H2S = 0.005 (1) Moles of KOH = 0.010/moles of KOH = 2 × moles of H2S (1) Volume = 66.7 cm³ (1)
[2] 1 mark each for any two of: • High melting point • Does not conduct electricity as a solid • Soluble in water • Conducts electricity as a molten liquid
[2] Magnesium (atom) loses 2 electrons (1) Sulfur (atom) gains 2 electrons (1)

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