Ten original multiple-choice questions and three structured questions, with answers and full working, covering every content standard of the Form 4 mole-concept chapter, written in SPM style, never copied from past-year papers.
Attempt every question before you read the answer, and always write the full working, not just the final value. Relative atomic masses: H = 1, C = 12, N = 14, O = 16, Na = 23, Mg = 24, S = 32, Cl = 35.5, Ca = 40, Cu = 64, Zn = 65. Use the molar volume for the stated conditions. These are original questions in SPM style, not past-year papers.
Section A, Multiple choice
1. What is the relative molecular mass of calcium carbonate, CaCO3? A. 82 B. 100 C. 84 D. 116
Answer: B. 40 + 12 + 3(16) = 100. The three oxygen atoms contribute 48.
2. How many moles are present in 4.0 g of sodium hydroxide, NaOH? (M = 40 g mol−1) A. 0.1 B. 0.2 C. 0.4 D. 1.0
Answer: A. n = mass ÷ molar mass = 4.0 ÷ 40 = 0.1 mol.
3. What is the mass of 0.5 mol of carbon dioxide, CO2? (M = 44 g mol−1) A. 11 g B. 22 g C. 44 g D. 88 g
Answer: B. mass = moles × molar mass = 0.5 × 44 = 22 g.
4. What volume does 2 mol of any gas occupy at room conditions? A. 12 dm3 B. 22.4 dm3 C. 44.8 dm3 D. 48 dm3
Answer: D. volume = moles × molar volume = 2 × 24 = 48 dm3.
5. How many moles of gas are in 6 dm3 measured at room conditions? A. 0.25 B. 0.5 C. 2.5 D. 4
Answer: A. n = volume ÷ molar volume = 6 ÷ 24 = 0.25 mol.
6. Which sample contains the greatest number of molecules? A. 0.10 mol CO2 B. 0.20 mol H2 C. 0.15 mol O2 D. 0.05 mol NH3
Answer: B. The number of molecules is proportional to the number of moles, so the largest number of moles (0.20 mol) has the most molecules.
7. A compound contains 75% carbon and 25% hydrogen by mass. What is its empirical formula? A. CH2 B. CH3 C. CH4 D. C2H4
Answer: C. C = 75 ÷ 12 = 6.25; H = 25 ÷ 1 = 25; ratio 6.25 : 25 = 1 : 4, so CH4.
8. The empirical formula of a compound is CH2 and its relative molecular mass is 42. What is its molecular formula? A. CH2 B. C2H4 C. C3H6 D. C4H8
Answer: C. Empirical formula mass = 14; n = 42 ÷ 14 = 3; molecular formula = C3H6.
9. In the reaction 2Mg + O2 → 2MgO, how many moles of magnesium oxide form from 0.4 mol of magnesium? A. 0.2 B. 0.4 C. 0.8 D. 1.0
Answer: B. The mole ratio Mg : MgO is 2 : 2 = 1 : 1, so 0.4 mol of Mg gives 0.4 mol of MgO.
10. How many moles of oxygen atoms are present in 0.5 mol of carbon dioxide, CO2? A. 0.5 B. 1.0 C. 1.5 D. 2.0
Answer: B. Each CO2 molecule contains 2 oxygen atoms, so moles of O atoms = 0.5 × 2 = 1.0 mol.
Section B, Structured questions
Structured question 1
A sample of ammonium sulfate, (NH4)2SO4, is used as a fertiliser. (a) Define relative molecular mass. (b) Calculate the relative molecular mass of ammonium sulfate. (c) Calculate the number of moles in 6.6 g of ammonium sulfate.
Model answer. (a) The relative molecular mass of a substance is the sum of the relative atomic masses of all the atoms shown in its formula, compared with one-twelfth of the mass of a carbon-12 atom; it has no unit. (b) Mr = 2(14 + 4×1) + 32 + 4(16) = 2(18) + 32 + 64 = 36 + 96 = 132. (c) n = mass ÷ molar mass = 6.6 ÷ 132 = 0.05 mol.
Structured question 2
Magnesium reacts with dilute hydrochloric acid according to the equation Mg + 2HCl → MgCl2 + H2. A student reacts 2.4 g of magnesium completely with excess acid. (a) Calculate the number of moles of magnesium used. (b) State the number of moles of hydrogen gas produced. (c) Calculate the volume of hydrogen gas produced, measured at room conditions.
Model answer. (a) n(Mg) = mass ÷ molar mass = 2.4 ÷ 24 = 0.1 mol. (b) From the equation, the mole ratio Mg : H2 is 1 : 1, so n(H2) = 0.1 mol. (c) volume = moles × molar volume = 0.1 × 24 = 2.4 dm3.
Structured question 3
A hydrocarbon contains 85.7% carbon and 14.3% hydrogen by mass. Its relative molecular mass is 56. (a) Determine the empirical formula of the hydrocarbon. (b) Determine its molecular formula.
Model answer. (a) Moles of C = 85.7 ÷ 12 = 7.14; moles of H = 14.3 ÷ 1 = 14.3. Divide by the smaller value (7.14): C = 1, H = 2.0. The empirical formula is CH2. (b) Empirical formula mass of CH2 = 12 + 2(1) = 14. n = Mr ÷ empirical formula mass = 56 ÷ 14 = 4. The molecular formula is C4H8.
Paper 1 technique
Paper 1 rewards speed and elimination. Three habits raise your score. First, work out the answer before you look at the options, then match it, this stops a distractor from misleading you. Second, use units to eliminate: an answer to “how many moles” cannot carry the unit dm3, so any option with the wrong kind of value is out. Third, estimate. If 0.5 mol of a gas must occupy about 12 dm3, an option of 48 dm3 is clearly wrong without a full calculation. Because Paper 1 is an objective (multiple-choice) paper, every question has exactly one correct option and there are no method marks, so a quick sanity check on each answer is worth the few seconds it takes.
How to mark yourself
Award yourself the method marks, not just the answer. In every structured question the marking scheme rewards the correct formula, the substitution with units, and the final answer with its unit, so if your final number is wrong but the method is right, you would still have gained most of the marks, and you can see exactly where the slip happened. Go back to any question you missed and identify which step failed: a wrong molar mass, a mole ratio the wrong way up, or a unit dropped. A one-to-one teacher can set you a short, targeted set of questions on just that step until it is secure. When you can complete all thirteen questions here with clean working, revisit the worked examples once more and then move on, because the same skills, convert to moles, use the ratio, convert back, carry straight into the acids and bases and electrochemistry chapters of SPM Chemistry.
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