Topic 2: Atoms, Molecules and Stoichiometry

Question 1

Definitions I:
(a) Relative atomic mass
(b) Mole
(c) Unified atomic mass unit

Answer 1

Definitions I:
(a) Avg. mass of an atom compared to 1/12ᵗʰ the mass of a ¹²C atom.
(b) Amount of substance with as many particles as atoms in 12 g ¹²C.
(c) Mass equal to 1/12ᵗʰ the mass of a ¹²C atom.

Question 2

Definitions II:
(a) Empirical formula
(b) Molecular formula
(c) Molar mass

Answer 2

Definitions II:
(a) Simplest whole-number ratio of atoms in a compound.
(b) Actual number of atoms per element in a compound’s molecule.
(c) Sum of relative atomic masses in a compound’s formula, relative to 1/12ᵗʰ of ¹²C.

Question 3

Definitions III:
(a) Anhydrous
(b) Water of crystallisation
(c) Limiting reagent

Answer 3

Definitions III:
(a) Compound without water of crystallisation, e.g., dry CuSO₄ after heating.
(b) Water molecules included in the crystalline structure of a hydrated compound.
(c) Reactant that is completely consumed, limiting the amount of product formed.

Question 4

What is the relative molecular mass of:
(a) H₂SO₄? [1 Mark]
(b) Na₂CO₃? [1 Mark]

Answer 4

(a) H₂: 2, S: 32, O₄: 64, total 98 (1 mark)
(b) Na₂: 46, C: 12, O₃: 48, total 106 (1 mark)

Question 5

How many moles are in 32 g CH₄ and what mass and volume of CO₂ does it produce on burning? [3 Marks]

Answer 5

Moles CH₄: 32/16 = 2 mol (1 mark)
Mass CO₂: 2 × 44 = 88 g (1 mark)
Volume CO₂: 2 × 24 = 48 dm³ (1 mark)

Question 6

What is the formula of:
(a) Aluminium phosphate? [1 Mark]
(b) Iron(III) sulfate? [1 Mark]

Answer 6

(a) AlPO₄ (1 mark)
(b) Fe₂(SO₄)₃ (1 mark)

Question 7

What mass of H₂O forms from 8 g H₂ reacting with excess O₂? [2 Marks]

Answer 7

Moles H₂: 8/2 = 4 mol (1 mark)
Mass H₂O: 4 × 18 = 72 g (1 mark)

Question 8

What volume of CO₂ at RTP is produced by burning 16 g CH₄? [2 Marks]

Answer 8

Moles CH₄: 16/16 = 1 mol (1 mark)
Volume CO₂: 1 × 24 = 24 dm³ (1 mark)

Question 9

What volume of 0.1 mol dm⁻³ HCl neutralises 20 cm³ of 0.2 mol dm⁻³ NaOH, and what is the percentage yield if 0.003 mol NaCl forms? [3 Marks]

Answer 9

Moles NaOH: 0.02 × 0.2 = 0.004 mol, volume HCl: 0.04 dm³ (2 marks)
Yield: (0.003/0.004) × 100 = 75% (1 mark)

Question 10

What is the ionic equation for Fe + CuSO₄? [2 Marks]

Answer 10

Fe(s) + Cu²⁺(aq) → Fe²⁺(aq) + Cu(s) (2 marks)

Question 11

What is the relative formula mass of FeSO₄·7H₂O and why is it hydrated? [2 Marks]

Answer 11

Fe: 56, S: 32, O₄: 64, 7H₂O: 126, total 278 (1 mark)
Water of crystallisation present (1 mark)

Question 12

What mass of O₂ reacts with 4 g H₂, and which is the limiting reagent? [2 Marks]

Answer 12

Moles H₂: 4/2 = 2 mol, O₂ mass: 1 × 32 = 32 g (1 mark)
H₂ limits, O₂ in excess (1 mark)