Law of Chemical Combination – Mole Concept

In this post, we will learn the laws of chemical combination with JEE PYQs, which underlie the mole concept. The laws of chemical combination form the foundation of chemistry and help us understand how substances react and combine to form new products. These laws explain the fixed relationships between elements in compounds and show that chemical reactions follow definite rules. From the conservation of mass to the concept of definite proportions, these principles make it easier to study and predict chemical changes.

Laws of Chemical Combination in Chemistry

• Law of Mass Conservation
• Law of Definite/Constant Proportion
• Law of multiple proportions
• Gay-Lussac’s law of gaseous volume
• Law of Reciprocal Proportion

Law of Mass Conservation

The law was proposed by the French chemist Antoine Laurent Lavoisier. According to this law, in any physical or chemical change, mass can neither be created nor destroyed.

Total mass of reactant = Total mass of product
For example: CaCO₃ (100g) → CaO (60g) + CO₂ (x)
Then, the value of x = 40g.

Note – This law does not apply to nuclear reactions.
E = Mc²

Question – If 12.6g of NaHCO₃ are added to 30g of CH₃COOH solution, the residue is found to weigh 36g. What is the mass of CO₂ released in the reaction?

Answer – NaHCO₃ + CH₃COOH → CH₃COONa + H₂O + CO₂
Amount Present – NaHCO₃ = 12.6g
CH₃COOH = 30g
CH₃COONa + H₂O = 36g
CO₂ = xg

12.6 + 30 = 36 + x
x = 6.6g

Law of Definite/Constant Proportion

The law was proposed by the French chemist Joseph Proust. Any given chemical compound always contains the same proportion of elements by mass.

For Example: H₂O [2:16], CO₂ [12:32], NH₃ [14:3]

Mass Percent of elements remains the same.

In H₂O [2:16],
Mass Per cent of H = Mass of H/Total Mass x 100%
H = 2/2+16 x 100%
H = 11.11%
Mass Per cent of O = Mass of O/Total Mass x 100%
O = 16/2+16 x 100%
O = 88.89%

Limitations:

• Not applicable to isotops.
• Not applicable for non-stoichiometric compounds.

Question – Hydrogen and Oxygen combine in a ratio of 1:8 by mass to form water. What mass of oxygen gas would be required to react completely with 3g of hydrogen gas?

Answer – H₂O [2:16] = [1:8]
1g H = 8g O
3g H = 8x3g O
3g H = 24g O

Law of multiple proportions

The law was proposed by the English chemist John Dalton. According to this law, when two elements combine to form two or more compounds, the masses of one of the elements combine with a fixed mass of the other, always bear a simple whole number ratio.

For Example:
C + O = CO [12:16]
C + O = CO₂ [12:32]
Mass ratio of O is [1:2]

H + O = H₂O [2:16]
H + O = H₂O₂ [2:32]
Mass ratio O is [1:2]

Gay-Lussac’s law of gaseous volume

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

For Example:
H₂ (50 ml) + Cl₂ (50 ml) = 2HCl (100 ml)
Volume ratio = 50:50:100 = 1:1:2

Law of Reciprocal Proportion

The law was proposed by Jeremias Ritcher in 1792. If two different elements B and C combine separately with the same weight of a third element A, the ratio of the masses in which they do so is either the same or a simple multiple of the mass ratio in which they combine B and C.

For Example: