How is the mole concept examined in DSE Chemistry?

The mole links mass, particle number and gas volume. Questions typically convert between mass and moles, then use the coefficients of a balanced equation to work out amounts of reactants and products.

Why must volume be converted to dm3?

Concentration is measured in mol per dm3, so a volume in cm3 must be divided by 1000 first. On dilution the number of moles of solute stays the same while the volume grows, so the concentration falls.

S4 Chemistry: Mole Concept and Concentration (Challenge, 10 Questions)

2026-11-06

Tutor Prep Guide · S4 Chemistry

Ten challenge items on limiting reagents, back-calculating concentration from a titration, percentage yield, molar mass from gas density, mass relations in decomposition, molecular formula from molar mass and percentages, and dilution, each with an answer and a note.

Quick answer Answers in order: 1.2 dm3, 0.0625 mol/dm3, 75 per cent, 32 g/mol, 28 g, 0.2 mol/dm3, 1.204 x 10 to the 24, C2H4O2, 1.12 dm3, 0.1 mol/dm3 with the amount unchanged. Compare required moles to find the limiting reagent.

01 Paper details (check answers afterwards)

Item	Detail
Questions	10 questions
Suggested time	about 45 minutes
Difficulty mix	Challenge 10
Chapter	Chapter: S4 Chemistry: Mole Concept and Concentration
Level	S2

02 Questions (10 items)

Challenge2.4 g of magnesium reacts with 100 cm3 of 1 mol/dm3 hydrochloric acid: Mg + 2HCl gives MgCl2 + H2. Find the volume of hydrogen at room conditions.
Challenge25.0 cm3 of 0.100 mol/dm3 sodium hydroxide exactly neutralises 20.0 cm3 of sulfuric acid: 2NaOH + H2SO4 gives Na2SO4 + 2H2O. Find the acid concentration.
ChallengeA reaction should give 0.2 mol of product of molar mass 60 g/mol but only 9 g is obtained. Find the percentage yield.
ChallengeAt room conditions with molar volume 24 dm3/mol, 1.2 dm3 of a gas weighs 1.6 g. Find its molar mass.
ChallengeCalcium carbonate decomposes on heating: CaCO3 gives CaO + CO2. Find the mass of calcium oxide from 50 g of calcium carbonate, taking the molar mass of CaO as 56.
Challenge12.5 g of CuSO4.5H2O, molar mass 250, is dissolved to make 250 cm3 of solution. Find the concentration of copper ions.
ChallengeFind the number of oxygen atoms in 0.5 mol of sulfuric acid, in units of 10 to the 23, taking the Avogadro constant as 6.02 x 10 to the 23 per mol.
Challenge0.1 mol of a compound weighs 6.0 g and contains 40.0 per cent carbon, 6.7 per cent hydrogen and 53.3 per cent oxygen by mass. Find its molecular formula.
ChallengeAt standard conditions the molar volume is 22.4 dm3/mol. Find the volume of 0.05 mol of gas at those conditions.
Challenge10 cm3 of 1 mol/dm3 solution is diluted with water to 100 cm3. Find the new concentration and state what happens to the amount of solute.

03 Answers and one-line notes

#	Answer	One-line note
1	1.2 dm3	The acid is limiting because 0.1 mol of magnesium needs 0.2 mol of acid, so only 0.05 mol of hydrogen forms, giving 1.2 dm3.
2	0.0625 mol/dm3	There are 0.0025 mol of hydroxide, so the acid is 0.00125 mol, and dividing by 0.020 dm3 gives 0.0625 mol per dm3.
3	75%	The theoretical mass is 12 g, so the yield is 9 / 12 = 75 per cent.
4	32 g/mol	The gas is 1.2 / 24 = 0.05 mol, so its molar mass is 1.6 / 0.05 = 32 g/mol.
5	28 g	0.5 mol of carbonate gives 0.5 mol of oxide by the 1 : 1 ratio, so the mass is 0.5 x 56 = 28 g.
6	0.2 mol/dm3	The crystals are 0.05 mol and each unit carries one copper ion, so 0.05 / 0.25 = 0.2 mol per dm3.
7	1.204 x 10 to the 24	Four oxygen atoms per molecule give 2 mol of oxygen atoms, which is 1.204 x 10 to the 24 atoms.
8	C2H4O2	The molar mass is 60 and the empirical formula CH2O has molar mass 30, so the multiplier is 2 and the formula is C2H4O2.
9	1.12 dm3	0.05 x 22.4 = 1.12 dm3, and the identity of the gas does not matter at fixed conditions.
10	0.1 mol/dm3, the amount of solute is unchanged	The 0.010 mol of solute stays the same while the volume becomes 0.1 dm3, so the concentration falls to 0.1 mol per dm3.

04 What to do after marking

After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
Repeat the same item two days later; move to the next paper only when both attempts are correct.
Parents only need the answer and note columns; there is no need to explain every item.

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