NDA Chemistry · Teaching notes

Mole Concept and Stoichiometry — NDA Chemistry

This is the one calculate-it chapter of NDA Chemistry — small in question count (9 PYQs across 2017–2026) but the only place the paper asks you to do arithmetic rather than recall a fact. Almost every question reduces to one habit: convert whatever you are given (grams, litres at STP, a number of molecules) into MOLES first, then convert moles into whatever the question wants. Get the mole bridge right and the chapter is free marks. It teaches in two movements: (1) Mole concept, Avogadro's law and molar calculations — the mole as a counting unit, Avogadro's number, molar mass, the three conversions (mass, particle count, volume at STP) and mass-percent; (2) Stoichiometry and the laws of chemical combination — reading mole ratios off a balanced equation, equivalent weight, and the named laws (conservation of mass, definite and multiple proportions, Avogadro's law). Mostly formula concepts with worked numbers; the named laws live in one reference table. Every PYQ tagged.

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Subtopic notes

PYQ weightage by concept

8 concepts · 9 PYQs — where the marks actually sit, so you know what to drill first

The Mole, Avogadro's Law and Molar Calculations5 PYQs · 56%

Concept	PYQs	Share
Avogadro's law and molar volume at STP	2	22%
Molar mass and moles from mass	1	11%
Counting particles from moles	1	11%
Mass-percent composition	1	11%
The mole and Avogadro's numberfoundation	—	—

Stoichiometry and the Laws of Chemical Combination4 PYQs · 44%

Concept	PYQs	Share
Laws of chemical combination	2	22%
Mole ratios from a balanced equation	1	11%
Equivalent weight and n-factor	1	11%

Formula & revision sheet

7 formulas · 1 reference tables · 10 gotchas across all subtopics — the exam-eve cheat-sheet

The Mole, Avogadro's Law and Molar Calculations

Formulas (5)

The mole and Avogadro's number · Avogadro's number $N_A = 6.022 \times 10^{23}\ \text{particles per mole}$
Molar mass and moles from mass · Moles from mass $n = \dfrac{m}{M}$
Avogadro's law and molar volume at STP · Moles from gas volume at STP $n = \dfrac{V}{22.4}$
Counting particles from moles · Particles from moles $N = n \, N_A = \dfrac{m}{M}\, N_A$
Mass-percent composition · Mass percent of an element $\%\,\text{element} = \dfrac{a \times A}{M} \times 100$

Watch out for (6)

Molecules and atoms differ for diatomic gases
→ The mole and Avogadro's number
Use the molar mass of the WHOLE molecule
→ Molar mass and moles from mass
22.4 L only at STP, and only for gases
→ Avogadro's law and molar volume at STP
Half a mole of a gas = 11.2 L (this is correct)
→ Avogadro's law and molar volume at STP
4 g of H2 is TWO Avogadro numbers, not one
→ Counting particles from moles
Mass-percent ratio ignores the oxygen count
→ Mass-percent composition

Stoichiometry and the Laws of Chemical Combination

Formulas (2)

Mole ratios from a balanced equation · Mass of product from mass of reactant $m_{\text{product}} = \dfrac{m_{\text{reactant}}}{M_{\text{reactant}}}\times \text{(mole ratio)} \times M_{\text{product}}$
Equivalent weight and n-factor · Equivalent weight $E = \dfrac{M}{n\text{-factor}}$

Reference tables (1)

Laws of chemical combination4 rows

Law	Statement	Stock example
Law of conservation of mass	Matter can neither be created nor destroyed in a chemical reaction; total mass of reactants = total mass of products.	1.7 g AgNO3 + 0.585 g NaCl produce 1.435 g AgCl + 0.85 g NaNO3 (masses balance both sides).Q By far the most-asked law in this chapter; any reaction where the two sides' masses add up to the same total is illustrating this law.
Law of definite (constant) proportions	A given pure compound always contains the same elements in the same fixed proportion by mass.	Water is always 1 : 8 hydrogen to oxygen by mass, whatever its source.
Law of multiple proportions	If two elements form more than one compound, the masses of one combining with a fixed mass of the other are in a ratio of small whole numbers.	Carbon + oxygen: CO and CO2 — the oxygen masses per fixed carbon are in a 1 : 2 ratio.
Avogadro's law	Equal volumes of all gases at the same temperature and pressure contain an equal number of molecules.	22.4 L of any gas at STP contains one mole (6.022 x 10^23 molecules). Also the basis for the 22.4 L molar volume used in the previous subtopic.

Recognise the law from either its definition or a worked mass-balance example.

Watch out for (4)

Coefficients are moles, not grams
→ Mole ratios from a balanced equation
Include the water of crystallisation in the molar mass
→ Equivalent weight and n-factor
Mass balancing means conservation of mass, not definite proportions
→ Laws of chemical combination
Definite vs multiple proportions
→ Laws of chemical combination