ICSE Class 10 Mole Concept — The ICSE-Style Questions
ICSE's Class 10 "Mole Concept and Stoichiometry" chapter (Selina and the ICSE syllabus) tests the mole idea in a distinctly different style from CBSE — vapour density questions, gas-volume-at-STP problems and formula-mass numericals appear far more often here than the molarity/solution-concentration style CBSE favours at the Class 11 level. This guide covers the mole concept exactly the way ICSE examines it, with every calculation worked in full.
Gram atomic, molecular and formula mass
ICSE is precise about terminology: the gram atomic mass is the atomic mass of an element expressed in grams (e.g. one gram atom of oxygen = 16 g); the gram molecular mass is the molecular mass expressed in grams for a molecular substance (e.g. one gram molecule of O₂ = 32 g); the gram formula mass is used for ionic compounds that do not exist as discrete molecules (e.g. NaCl).
Vapour density — the ICSE-specific relation
Vapour density (VD) is defined as the ratio of the mass of a certain volume of a gas to the mass of an equal volume of hydrogen gas, measured under the same conditions of temperature and pressure. Because hydrogen's molecular mass is 2, this gives a very useful shortcut:
This relation, built on Avogadro's law (equal volumes of gases at the same temperature and pressure contain equal numbers of molecules), is asked constantly in ICSE papers and rarely appears in the CBSE Class 11 treatment of the mole concept, which leans instead on molarity and percentage-composition problems.
Worked example 1 — molecular mass from vapour density
A gas has a vapour density of 22. Find its molecular mass and suggest what the gas could be.
Molecular mass = 2 × VD = 2 × 22 = 44 g/mol — this matches carbon dioxide, CO₂ (12.011 + 2 × 15.999 = 44.01 g/mol).
Molar volume — which STP convention applies
Older ICSE and most current Indian school textbooks use the classical STP (0°C, 1 atmosphere), at which one mole of any gas occupies 22.4 litres. IUPAC's current recommended STP is 0°C and 1 bar (a very slightly lower pressure), which gives 22.7 litres per mole instead. Use whichever value your own textbook or exam paper states; 22.4 L/mol remains the figure almost universally used in ICSE-level numericals, and this guide follows that convention.
Worked example 2 — moles, particles and gas volume
How many moles are present in 4.4 g of CO₂? How many molecules is that, and what volume would it occupy at STP?
Moles = mass ÷ molar mass = 4.4 ÷ 44 = 0.1 mol
Number of molecules = 0.1 × 6.022 × 10²³ = 6.022 × 10²² molecules
Volume at STP = 0.1 mol × 22.4 L/mol = 2.24 L
Worked example 3 — volume of a gas at STP
What volume does 0.5 mol of oxygen gas occupy at STP?
Volume = moles × molar volume = 0.5 × 22.4 = 11.2 litres
Worked example 4 — empirical formula from percentage composition
A compound is found to contain 40% carbon, 6.7% hydrogen and 53.3% oxygen by mass. Find its empirical formula.
Assume 100 g of compound, so the percentages become masses directly.
Moles C = 40 ÷ 12.011 = 3.331
Moles H = 6.7 ÷ 1.008 = 6.647
Moles O = 53.3 ÷ 15.999 = 3.332
Divide every value by the smallest (3.331):
C: 3.331 ÷ 3.331 = 1.00
H: 6.647 ÷ 3.331 = 2.00
O: 3.332 ÷ 3.331 = 1.00
Empirical formula = CH₂O (ratio 1 : 2 : 1)
Common mistakes that cost marks
- Forgetting to divide by the smallest mole value. The empirical formula method only gives a correct whole-number ratio after this step — skipping it leaves decimal subscripts that make no chemical sense.
- Using VD = molecular mass (without dividing by 2). Vapour density is always relative to hydrogen, so the factor of 2 must not be dropped.
- Mixing gram atomic mass and gram molecular mass. One gram atom of oxygen (16 g) is not the same as one gram molecule of O₂ (32 g) — a common ICSE trap.
- Rounding mole values too early before dividing by the smallest value, which can turn a clean 1:2:1 ratio into an incorrect one.
Where mole concept appears in ICSE exams
| Question style | What is tested |
|---|---|
| Vapour density numericals | Converting VD to molecular mass and identifying the gas |
| Gas volume at STP/NTP | Mole–volume relationship using the molar volume constant |
| Formula-mass calculations | Gram atomic/molecular/formula mass, often combined with equation-based problems |
| Empirical/molecular formula | Percentage composition converted to a whole-number atom ratio |
Check formula masses instantly. The free calculator suite's Molar Mass & Composition tool accepts any formula and shows the element-wise breakdown, useful for checking your own vapour-density and empirical-formula workings.
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