The questions are on the mole calculations test. Take it under real conditions before reading the mark scheme.
Quick answer: The mole is the chemist's counting unit: one mole contains 6.02 × 10²³ particles, the Avogadro constant. This mark scheme gives the model answers and the marks for the 8 questions of the Chemistry 9-10 test on mole calculations, from Avogadro constant to chloride ions, with the equivalent answers to accept.
Score your paper out of 20 with the model answers and marking points. If you lost marks on a calculation, redo it using units at each step and check whether you used the correct molar mass for the whole particle.
Level: Chemistry grades 9-10 (GCSE · IGCSE · O Level · Class 9-10) · Curriculum: HS-PS1-7, GCSE C3
Question 1 Avogadro constant (2 marks)
It is 6.02 × 10²³ per mole, the number of particles in one mole of a substance.Marking: 1 mark for the value, 1 mark for the meaning.
Question 2 Aluminium nitrate (3 marks)
- 27 + 3 × (14 + 48) = 27 + 186 = 213.
- 21.3 ÷ 213 = 0.100 mol.
Marking: 1 mark for the bracket method, 1 mark for 213, 1 mark for 0.100 mol.
Question 3 Mass of copper oxide (3 marks)
M = 63.5 + 16 = 79.5 g/mol. m = n × M = 0.40 × 79.5 = 31.8 g.Marking: 1 mark for M, 1 mark for the formula, 1 mark for 31.8 g.
Question 4 Molecules in water (3 marks)
M = 18 g/mol. n = 3.6 ÷ 18 = 0.20 mol. N = 0.20 × 6.02 × 10²³ = 1.20 × 10²³ molecules.Marking: 1 mark for M, 1 mark for n, 1 mark for N.
Question 5 Oxygen error (2 marks)
Oxygen gas is made of O₂ molecules, so the molar mass is 32 g/mol, not 16. The correct amount is 8.0 ÷ 32 = 0.25 mol.Marking: 1 mark for the molar mass of O₂, 1 mark for 0.25 mol.
Question 6 Moles in 10 g (3 marks)
- Water.
- Water has the smallest molar mass (18 g/mol), so 10.0 g is the largest number of moles.
- 0.556 × 6.02 × 10²³ = 3.34 × 10²³ molecules.
Marking: 1 mark for each part.
Question 7 Nitrogen in fertiliser (2 marks)
Mr = 14 + 4 + 14 + 48 = 80. Nitrogen mass = 28. Percentage = 28 ÷ 80 × 100 = 35.0 %.Marking: 1 mark for Mr = 80, 1 mark for 35.0 %.
Question 8 Chloride ions (2 marks)
Each CaCl₂ gives two Cl⁻ ions, so 2 × 0.30 = 0.60 mol.Marking: 1 mark for the factor of 2, 1 mark for 0.60 mol.
