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Playbook

Mole Concept and Stoichiometry

9 q · 11% HARD — the bank's only Calculate-strand chapter. Mole = 6.022×10²³ particles; molar mass arithmetic; Avogadro's law (equal V → equal n at same T, P); stoichiometric ratios from balanced equations; equivalent weight = molar mass / valency factor; laws of chemical combination (conservation of mass, definite proportions).

questions in the bank
9
tagged HARD
11%
subtopic(s)
2
worked examples
2

When you’ll see it

A mass↔mole conversion, an Avogadro-particle count, an equivalent-weight calculation, or a 'which law of chemical combination is shown' question.

How this chapter is tested

9 q in 10 years · 11% HARD — the bank's only Calculate-strand chapter. The 4 core formulas: mol = mass / molar mass; mol = particles / N_A where N_A = 6.022×10²³; mass(g) = mol × molar mass; equiv-weight = molar mass / valency factor. Worked example: mass of 0.5 mol N₂. Molar mass N₂ = 28 g/mol. mass = 0.5 × 28 = 14 g. That's the standard MOD plug-in.

Equivalent weight is the tricky form. For an acid: equiv-weight = molar mass / basicity (number of replaceable H⁺). HCl basicity 1 → equiv 36.5. H₂SO₄ basicity 2 → equiv 49. H₃PO₄ basicity 3 → equiv 32.7. For a base: / acidity (number of OH⁻). For a salt: / total positive charge. Oxalic acid C₂H₂O₄·2H₂O: molar mass = 24 + 2 + 64 + 36 = 126 g/mol; basicity 2 → equiv 63 (the classic NDA 2019 question).

Stoichiometry questions in NDA are typically 'which law is shown': Conservation of mass (Lavoisier — total mass before = total mass after; e.g. 1.7 g AgNO₃ + 0.585 g NaCl → 1.435 g AgCl + 0.85 g NaNO₃, total 2.285 g both sides). Definite proportions (Proust — a pure compound always has the same elemental ratio by mass). Multiple proportions (Dalton — when same elements form 2+ compounds, the masses of B per fixed mass of A are in simple ratios; e.g. CO vs CO₂ have O:C ratios 16:12 vs 32:12, ratio 1:2). Match the data pattern to the law — don't try to derive.

The sub-skills

The rules and habits that decide whether you get a question right.

  • Mass ↔ mole conversion

    mol = mass(g) / molar mass(g/mol). mass(g) = mol × molar mass. For ratios, the moles cancel — only the mass-to-mass ratio matters once balanced. Watch units: kg ↔ g, mL ↔ L.

  • Avogadro particle counting

    mol = particles / N_A where N_A = 6.022×10²³. 1 mol H₂O contains N_A molecules of H₂O, but 2N_A H atoms + N_A O atoms (count atoms not molecules when asked). 22.4 L at STP = 1 mol gas (Avogadro's law).

  • Equivalent weight calculation

    Acid: molar mass / basicity (replaceable H⁺ count). HCl/1, H₂SO₄/2, H₃PO₄/3, CH₃COOH/1, H₂C₂O₄/2 (oxalic acid). Base: / acidity (OH⁻ count). NaOH/1, Ca(OH)₂/2, Al(OH)₃/3. Salt: / total positive charge. Na₂CO₃: 106/2 = 53.

  • Law-of-combination matching

    Mass conservation: total mass in = total mass out. Definite proportions: fixed elemental mass ratio in a pure compound (H₂O always 1:8 H:O by mass). Multiple proportions: 2+ compounds of same elements have mass ratios in simple integer ratios (CO vs CO₂; H₂O vs H₂O₂).

2 worked examples from the bank

Real past-year questions illustrating the playbook. Click to reveal options + solution.

Example 1Mole Concept and StoichiometryHARD
The compound C6H12O4\text{C}_6\text{H}_{12}\text{O}_4 contains

[Q84 · Sep · 2017]

Example 2Mole Concept and StoichiometryMODERATE
The equivalent weight of oxalic acid in C2H2O4⋅2H2O\text{C}_2\text{H}_2\text{O}_4\cdot 2\text{H}_2\text{O} is

[Q86 · Apr · 2019]

Traps to expect

Distractor shapes specific to this chapter. The page-wide Traps section covers the bank-level patterns.

  • Mass of N₂ vs mass of N atoms

    0.5 mol N₂ has mass 0.5 × 28 = 14 g (molar mass of N₂ molecule = 28). If the question asked for mass of N ATOMS in 0.5 mol N₂ (= 1 mol N atoms), it'd be 14 g (still equal because 1 mol N × 14 = 14). But for fractional cases — 0.5 mol N₂ contains 1 mol N atoms — count atoms not molecules.

  • Equivalent weight of polyprotic acids

    For H₂SO₄ (diprotic), equiv = 98/2 = 49 (not 98). For H₃PO₄ (triprotic), equiv = 98/3 = 32.7 (not 98). Wrong option uses molar mass directly without dividing by basicity.

  • STP volume only for IDEAL gas

    1 mol gas at STP = 22.4 L applies only to IDEAL gases (or close-to-ideal at low pressure, normal T). Real gases deviate. NDA generally tests ideal-gas only — but watch for 'at STP, 1 mol of ANY substance = 22.4 L' (false for solids/liquids).

Drill every mole concept and stoichiometry question

9 questions from the bank, scoped to 2 bundled subtopics.

Drill the 9 questions

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