When water is added to magnesium nitride, Mg3N2, the products are a white suspension of Mg(OH)2 and an alkaline gas.

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2017). The mark or mark type is shown in brackets.

1. [1] Mg3N2 + 6H2O → 3Mg(OH)2 + 2NH3
2. [1] moles of Mg3N2 = 2.52 / 100.9 = 0.025 (0.0249)
3. [1] (moles of Mg(OH)2 = 0.075 (0.0749))
4. [1] mass of Mg(OH)2 = (0.075 × 58.3) = 4.37 g or 4.4 g
5. [1] solubility increases (down the group)
6. [1] ∆Hlatt and ∆Hhyd both decrease / less exothermic / more endothermic
7. [1] but ∆Hlatt decreases more (than ∆Hhyd decreases)
8. [1] ∆Hsol becomes more negative / more exothermic / less endothermic
9. [1] Ksp = [Mg²⁺][OH⁻]²
10. [1] Ksp = (1.7 x 10⁻⁴) × (2 × 1.7 × 10⁻⁴)² = 2.0 × 10⁻¹¹ (1.97 × 10⁻¹¹)
11. [1] mol³ dm⁻⁹
12. [1] cations become bigger / ionic radius increases
13. [1] polarisation/distortion of anion / hydroxide ion decreases