Laws of Chemical Combination | Basic Concepts of Chemistry Class 11

Laws of Chemical Combination

Foundation of Chemistry | Some Basic Concepts of Chemistry

1. Law of Conservation of Mass

Proposed by Antoine Lavoisier (1789).

                    Statement: Matter can neither be created nor destroyed. In a chemical reaction, the total mass of reactants is equal to the total mass of products.
                

Example: Combustion of Carbon.

$$ C (12g) + O_2 (32g) \rightarrow CO_2 (44g) $$

Total Mass Reactants ($12+32=44g$) = Total Mass Product ($44g$).

2. Law of Definite Proportions

Proposed by Joseph Proust.

                    Statement: A given compound always contains exactly the same proportion of elements by weight, irrespective of the source or method of preparation.
                

Example: Water ($H_2O$) obtained from a river, rain, or lab synthesis always contains Hydrogen and Oxygen in the mass ratio of 1:8.

3. Law of Multiple Proportions

Proposed by John Dalton (1803).

                    Statement: If two elements combine to form more than one compound, the masses of one element that combine with a fixed mass of the other element are in the ratio of small whole numbers.
                

Example: Carbon and Oxygen form $CO$ and $CO_2$.

In $CO$: 12g Carbon combines with 16g Oxygen.
In $CO_2$: 12g Carbon combines with 32g Oxygen.

Ratio of Oxygen masses (fixed Carbon): $16 : 32 \Rightarrow 1 : 2$ (Simple whole number ratio).

4. Gay Lussac's Law of Gaseous Volumes

Proposed by Gay Lussac (1808).

                    Statement: When gases combine or are produced in a chemical reaction, they do so in a simple ratio by volume, provided all gases are at the same temperature and pressure.
                

Example: Formation of HCl.

$$ H_2(g) + Cl_2(g) \rightarrow 2HCl(g) $$

Volume Ratio: 1 vol : 1 vol : 2 vol (Simple Ratio $1:1:2$).

5. Avogadro's Law

Proposed by Amedeo Avogadro (1811).

                    Statement: Equal volumes of all gases at the same temperature and pressure contain an equal number of molecules.
                

$$ V \propto n \quad (\text{at constant } T, P) $$

This explains why the molar volume of any ideal gas at STP is 22.4 L.

Practice Quiz

Test your understanding of Chemical Laws.

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