Laws of Chemical Combination and Mole Concept

Memory aid 'C-D-MM-RP-G': (1) Law of Conservation of Mass (Lavoisier) - matter is neither created nor destroyed. (2) Law of Definite/Constant Proportions (Proust) - a pure compound always has the same elements in the same fixed mass ratio. (3) Law of Multiple Proportions (Dalton) - when two elements form more than one compound, the masses of one combining with a fixed mass of the other are in small whole-number ratios (e.g., CO and CO2: O ratio 1:2). (4) Gay-Lussac's Law of Gaseous Volumes - gases combine in simple whole-number volume ratios at same T,P. (5) Avogadro's Law - equal volumes of gases at same T,P contain equal number of molecules. Avogadro's law resolved the atom-molecule confusion of Dalton's theory.

Avogadro number NA = 6.022 x 10^23. Number of moles can be found four ways: n = given mass/molar mass = number of particles/NA = volume at STP/22.4 L (gases) = molarity x volume(L). At STP (273.15 K, 1 bar) molar volume = 22.7 L; at 273.15 K and 1 atm it is 22.4 L. Mass of one atom = molar mass/NA. Number of molecules = n x NA; number of atoms = n x NA x (atomicity). Shortcut: 1 mole = molar mass in grams = 6.022 x 10^23 particles = 22.4 L gas at STP(1 atm). Remember total moles of atoms in a molecule = moles of molecules x number of atoms per molecule.

Carbon forms two oxides. In CO, 12 g C combines with 16 g O. In CO2, 12 g C combines with 32 g O. Fixing carbon mass at 12 g, the oxygen masses are 16 g and 32 g, giving ratio 16:32 = 1:2, a simple whole-number ratio - illustrating the Law of Multiple Proportions. Another example: H2O (2 g H : 16 g O) and H2O2 (2 g H : 32 g O) gives oxygen ratio 1:2. Tip: always fix the mass of one element, then take the ratio of the other element's masses to verify the law quickly in exams.

Cover the answer, recall, then check. 12 cards on the foundational laws and the mole.

Q1. State the Law of Conservation of Mass and name its proposer.
A1. Matter is neither created nor destroyed in a chemical reaction — total mass of reactants = total mass of products. Given by Antoine Lavoisier.

Q2. What does the Law of Definite (Constant) Proportions state?
A2. A given compound always contains the same elements in the same fixed mass ratio, regardless of source or method of preparation (Proust). E.g. water is always 1:8 H:O by mass.

Q3. State the Law of Multiple Proportions with an example.
A3. When two elements form more than one compound, the masses of one element combining with a fixed mass of the other are in a small whole-number ratio (Dalton). E.g. in CO and CO₂, O per fixed 12 g C is 16 g and 32 g → 1:2.

Q4. State Gay-Lussac's Law of Gaseous Volumes.
A4. When gases react, they do so in volumes bearing a simple whole-number ratio to one another and to the products, at the same temperature and pressure. E.g. H₂ + Cl₂ → 2HCl is 1:1:2 by volume.

Q5. State Avogadro's Law.
A5. Equal volumes of all gases at the same temperature and pressure contain equal numbers of molecules.

Q6. What is the value of Avogadro's number (Nₐ)?
A6. 6.022 × 10²³ per mole — the number of specified particles in one mole.

Q7. Define one mole.
A7. The amount of substance containing exactly 6.022 × 10²³ elementary entities (atoms, molecules, ions, electrons).

Q8. What volume does 1 mole of any ideal gas occupy at STP (0 °C, 1 bar)?
A8. 22.7 L (molar volume at 273.15 K and 1 bar, current NCERT). At the older 1 atm STP it is 22.4 L.

Q9. How many moles are in 11 g of CO₂?
A9. Molar mass CO₂ = 44 g/mol → 11/44 = 0.25 mol.

Q10. How many oxygen atoms are in 0.5 mol of H₂SO₄?
A10. Each molecule has 4 O atoms → 0.5 × 4 × 6.022×10²³ = 1.2044 × 10²⁴ O atoms.

Q11. Relate number of moles (n) to mass (m), molar mass (M), and particles (N).
A11. n = m/M = N/Nₐ = V/22.7 (for a gas at STP, V in litres).

Q12. Distinguish the three "definite/multiple" laws by what is held fixed.
A12. Definite proportions — same compound, fixed mass ratio. Multiple proportions — same two elements, different compounds, whole-number ratio of one element's masses. Reciprocal proportions — masses of two elements that combine with a fixed mass of a third combine among themselves in the same (or a simple multiple) ratio.

The mole is the single most-used idea in NEET Chemistry — almost every numerical in Physical Chemistry starts with "convert to moles." Master the five combination laws and the three mole bridges (mass, particles, gas volume) and you unlock stoichiometry, concentration, and gas problems.

The five laws of chemical combination

Law	Statement (one line)	Proposer
Conservation of Mass	Mass of reactants = mass of products	Lavoisier
Definite Proportions	A compound always has the same fixed mass ratio	Proust
Multiple Proportions	Masses of one element per fixed mass of another are in whole-number ratios	Dalton
Gay-Lussac (volumes)	Reacting gas volumes are in simple whole-number ratios	Gay-Lussac
Avogadro	Equal volumes of gases (same T, P) have equal molecules	Avogadro

The mole — three bridges

One mole = 6.022 × 10²³ entities = molar mass in grams = 22.7 L for an ideal gas at STP (273.15 K, 1 bar; 22.4 L at the older 1 atm STP).

• Mass bridge: n = m / M
• Number bridge: n = N / Nₐ
• Gas bridge: n = V / 22.7 (litres at STP)

These three are interchangeable — pick whichever the data gives you, convert to moles, then convert out.

Exam Tricks & Tips

• 🎯 Everything routes through moles. Never combine masses directly; convert to moles first, use the balanced-equation ratio, then convert back.
• 🎯 CO/CO₂ is the classic multiple-proportions case: 12 g C fixes 16 g vs 32 g O → 1:2. Expect one data-based question every session.
• 🎯 Gram atom vs gram molecule: "gram atom of oxygen" means 1 mol O atoms (16 g); "gram molecule" means 1 mol O₂ (32 g). Read carefully.
• 🎯 Use 22.7 L only at STP. For any other T or P use PV = nRT — do not force 22.7.
• 🎯 Atoms per formula unit trap: 1 mol H₂SO₄ has 2 mol H, 1 mol S, 4 mol O — multiply by Nₐ only after counting atoms in the formula.
• ❌ Common mistake: treating STP molar volume as 22.4 L in current NCERT numericals — the syllabus value is 22.7 L at 1 bar; check which the question intends.

Expected exam pattern

1–2 direct questions per paper: identifying which law a data set illustrates (multiple proportions is a favourite), or a one-step mole ↔ mass ↔ number ↔ volume conversion. Usually easy, quick marks.

Quick recap

Five laws: conservation, definite, multiple, Gay-Lussac, Avogadro. One mole = 6.022 × 10²³ = molar mass in g = 22.7 L gas at STP. Convert everything to moles first; the three bridges (m/M, N/Nₐ, V/22.7) do the rest.