The questions are on the ideal gas equation test. Take it under real conditions before reading the mark scheme.
Quick answer: The amount of substance, in mol, links mass, particle number, solution volume and gas volume. This mark scheme gives the model answers and the marks for the 8 questions of the Chemistry 11-12 test on ideal gas equation, from mass of potassium carbonate to why atom economy matters, with the equivalent answers to accept.
Marks are awarded for method and for the final answer with the correct unit. Allow error carried forward when an earlier slip is used consistently. Answers with correct numbers but no units lose one mark.
Level: Chemistry grades 11-12 (A Level · IB · Class 11-12) · Curriculum: A Level 3.1.2
Question 1 Mass of potassium carbonate (2 marks)
M = 2 × 39.1 + 12.0 + 3 × 16.0 = 138.2 g mol⁻¹. m = 0.0300 × 138.2 = 4.15 g.Marking: 1 for M; 1 for the mass with unit.
Question 2 Formula of an oxide of lead (3 marks)
n(Pb) = 2.07 ÷ 207 = 0.0100 mol; n(O) = 0.32 ÷ 16.0 = 0.0200 mol. Ratio 1 : 2. Formula PbO₂.Marking: 1 for each amount; 1 for the ratio and formula.
Question 3 Graph of gas volume (4 marks)
a. About 2.49 dm³ (accept 2.4 to 2.6).b. V is directly proportional to the kelvin temperature.
c. V = 3.32 × 10⁻³ m³, T = 400 K, p = 1.00 × 10⁵ Pa. n = pV ÷ RT = (1.00 × 10⁵ × 3.32 × 10⁻³) ÷ (8.31 × 400) = 332 ÷ 3324 = 0.100 mol.
Marking: (a) 1; (b) 1; (c) 1 for conversion, 1 for the result.
Question 4 Atom economy of oxygen (3 marks)
Total mass of products = 2 × 18.0 + 32.0 = 68.0. Atom economy = 32.0 ÷ 68.0 × 100 = 47.1 %, which is low, since more than half the mass goes into water.Marking: 1 for the total; 1 for the value; 1 for the comment.
Question 5 Molecular formula (2 marks)
Mass of CH₂ = 14.0. 84.0 ÷ 14.0 = 6. The molecular formula is C₆H₁₂.Marking: 1 for the factor 6; 1 for the formula.
Question 6 Volume of a gas (3 marks)
p = 2.00 × 10⁵ Pa. V = nRT ÷ p = (0.500 × 8.31 × 300) ÷ (2.00 × 10⁵) = 6.23 × 10⁻³ m³ = 6.23 dm³.Marking: 1 for conversion of p; 1 for substitution; 1 for the answer with unit.
Question 7 Percentage yield (2 marks)
Yield = 4.50 ÷ 6.00 × 100 = 75.0 %.Marking: 1 for the ratio; 1 for the answer.
Question 8 Why atom economy matters (1 mark)
Less waste is produced, so fewer by-products need disposal and fewer resources are used per tonne of useful product.Marking: 1 for less waste or fewer resources.
