Laws of chemical combinations

1. Law of conservation of mass: Given by Lavoisier: “In all chemical changes, the total mass of the system remains constant.”i.e., Matter can neither be created nor destroyed.

Ex: 2H2 + O2 = 2H2O.

Reactants mass = 4 + 32 = 36 g.

Products mass = 2 X (2 + 16) = 36 g.

Reactants mass = Products mass.

2. Law of definite or constant proportions: Given by Proust: “A given compound always contains its constituent elements in the same ratio by weight.”i.e., the method of preparation of a compound does not affect the combining ratio of the elements (weight).

3. Law of multiple proportions: Given by Dalton: “When two elements combine with each other to form more than one compound, the weights of one element that combine with a fixed weight of the other are in a ratio of small whole numbers”.

For example, there are five distinct oxides of nitrogen, and the weights of oxygen in combination with 14 grams of nitrogen are, in increasing order, 8, 16, 24, 32 and 40 grams, or in a ratio of 1, 2, 3, 4 and 5.

4. Law of gaseous volumes: Given by Gay-Lussac: “The volume of gases taking part in a chemical reaction show simple whole number ratios to one another when those volumes are measured at the same temperature and pressure”.

1 liter of nitrogen gas reacts with 3 litres of hydrogen gas to produce 2 litres of ammonia gas.

N2 (g) + 3H2 (g) → 2NH3 (g).

Since all the reactants and products are gases, the mole ratio of N2 (g): H2 (g): NH3 (g) of 1:3:2 is also the ratio of the volumes of gases.

So, 10 mL of nitrogen gas would react with 10 x 3 = 30 mL of hydrogen gas to produce 10 x 2 = 20 mL ammonia gas.

5. Avogadro’s law: Given by Avogadro: Under the same conditions of temperature and pressure, equal volumes of different gases contain an equal number of molecules.

The volume occupied by one gram-mole of gas is about 22.4 L (0.791 cubic feet) at standard temperature and pressure (0 °C, 1 atmosphere) and is the same for all gases, according to Avogadro’s law and they all contain 6.023 x 1023