Subtopic 1 — Laws of Chemical Combination

Five sequential laws form the historical basis of quantitative chemistry:

• Conservation of Mass (Lavoisier, 1789): In any chemical reaction, total mass of reactants = total mass of products. No matter is created or destroyed.
• Definite Proportions (Proust, 1799): A compound always contains the same elements in the same mass ratio. Water is always 1:8 (H:O by mass) regardless of source.
• Multiple Proportions (Dalton, 1803): When two elements form two different compounds (e.g., CO and $CO_{2}$), the masses of oxygen combining with a fixed mass of carbon are in a simple whole-number ratio (16:32 = 1:2).
• Gaseous Volumes (Gay-Lussac, 1808): Volumes of gases that react and are produced are in simple whole-number ratios at the same temperature and pressure.
• Avogadro's Law (1811): Equal volumes of all gases at the same T and P contain equal numbers of molecules.

Subtopic 2 — Mole Concept and Avogadro's Number

1 mol = $6.022 \times 10^{23}$ entities = molar mass in grams. The molar volume of any ideal gas at STP = 22.4 L. The three core interconversions are: mass ↔ moles (via molar mass), moles ↔ particles (via Nₐ), and moles ↔ volume at STP (via 22.4 L/mol).

Subtopic 3 — Percentage Composition and Formulae

% element = (n × atomic mass / molar mass) × 100. Empirical formula is derived from elemental percentages; molecular formula from EF and molar mass using n = Mr / EF mass. A classic NEET question provides % C, H, O and asks for both formulae.

Subtopic 4 — Stoichiometry and Limiting Reagent

Balanced equations give mole ratios. Steps: (1) balance the equation, (2) convert given data to moles, (3) identify limiting reagent (smallest moles/coefficient), (4) calculate product moles from limiting reagent, (5) convert to required units. Excess reagent and percentage yield calculations extend this base.

Subtopic 5 — Concentration Terms and Interconversions

Six concentration units (M, m, x, N, mass%, ppm) differ in temperature dependence and denominator. Mass-based units (m, x, mass%, ppm) are T-independent; volume-based units (M, N) are T-dependent. The two NEET-critical interconversion formulas link mass% and density to M and m. Equivalent weight = Mr / n-factor; N = M × n-factor.