Laws of Chemical Combination
~7 min read
- Five laws: Conservation of Mass (Lavoisier 1789), Definite Proportions (Proust 1799), Multiple Proportions (Dalton 1803), Reciprocal Proportions (Richter), Gay-Lussac's Law of Volumes.
- Avogadro's Hypothesis: Equal volumes of gases at same T and P contain equal numbers of molecules.
- Mole: SI base unit. 6.022 × 10²³ entities (Avogadro number).
The basic quantitative laws of chemistry. NDA tests them by name and example.
The Five Foundational Laws
- Conservation of Mass (Lavoisier, 1789): In a chemical reaction, total mass of reactants = total mass of products. No matter is created or destroyed.
- Definite (Constant) Proportions (Proust, 1799): A pure compound always contains the same elements in the same ratio by mass. Water (H₂O) is always 1:8 H:O by mass.
- Multiple Proportions (Dalton, 1803): When two elements combine to form more than one compound, the masses of one element combining with a fixed mass of the other are in small whole-number ratios. C + O → CO and CO₂; the O ratio is 1:2.
- Reciprocal Proportions (Richter): The ratios in which two elements combine separately with a third element are also the ratios in which they combine with each other (or in simple multiples).
- Gay-Lussac's Law of Combining Volumes: When gases react, the ratio of volumes (at same T, P) is a small whole number. 1 vol H₂ + 1 vol Cl₂ → 2 vol HCl.
Avogadro's Hypothesis (1811)
- "Equal volumes of all gases, at the same temperature and pressure, contain equal numbers of molecules."
- Initially controversial; later accepted (1858, Cannizzaro).
- Implies that ratios of gas volumes are ratios of molecular numbers.
- Resolved confusion between atoms and molecules.
The Mole and Avogadro's Number
- Mole: SI unit for amount of substance. 1 mol = 6.022 × 10²³ entities (atoms, molecules, ions, etc.).
- Avogadro's number (N_A): 6.022 × 10²³.
- Mass of 1 mol of a substance (in grams) = its molecular mass.
- 1 mol H = 1 g.
- 1 mol C = 12 g.
- 1 mol H₂O = 18 g.
- 1 mol CO₂ = 44 g.
- At STP (Standard Temperature and Pressure): 1 mol of any gas occupies 22.4 L.
NDA PYQ Examples
Q: Law of conservation of mass was given by:
(a) Dalton (b) Lavoisier (c) Proust (d) Avogadro
Answer: (b) Lavoisier — 1789.
Q: Water always contains H and O in mass ratio 1:8. This is law of:
(a) Conservation (b) Definite proportions (c) Multiple proportions (d) Combining volumes
Answer: (b) Definite proportions.
Q: At STP, 1 mole of any gas occupies:
(a) 1 L (b) 11.2 L (c) 22.4 L (d) 100 L
Answer: (c) 22.4 L.
Q: Avogadro's number is approximately:
(a) 3.14 × 10²³ (b) 6.022 × 10²³ (c) 9.8 × 10²³ (d) 1.6 × 10⁻¹⁹
Answer: (b) 6.022 × 10²³.
Drill Laws of Chemical Combination for NDA
NDA-pattern items on Laws of Chemical Combination with answer keys and explanations.
Start Free Mock TestFrequently Asked Questions
How was Avogadro's number determined?
Originally inferred from gas behaviour and atomic weights. Modern measurement uses X-ray crystallography of silicon spheres, electrochemistry of silver, and other methods. Defined as exactly 6.02214076 × 10²³ since 2019.
Why is the mole a useful unit?
It connects the atomic scale (single atoms/molecules) to laboratory scale (grams of substance). 1 mole of any substance contains the same number of particles. Lets us measure out exact stoichiometric ratios.
What does STP stand for?
Standard Temperature and Pressure. Conditions: 0°C (273.15 K) and 1 atm (101.325 kPa). Used as reference for gas calculations. NTP (Normal): 20°C, 1 atm — slightly different.
Why are gas volumes simpler than mass ratios?
Avogadro's hypothesis: equal volumes contain equal numbers of molecules at same T, P. So volume ratios directly give molecule ratios. Mass ratios depend on different molecular weights — more complex.
Is the mole defined in SI?
Yes — one of the seven base units. Redefined in 2019: the mole contains exactly 6.02214076 × 10²³ elementary entities. No longer tied to carbon-12 mass.