Laws of Chemical Combination

The Laws of Chemical Combination are fundamental principles that govern how elements combine to form compounds. These laws were established in the late 18th and early 19th centuries and form the basis of stoichiometry in chemistry.

1. Law of Conservation of Mass

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

Proposed by: Antoine Lavoisier (1774)

Mass of reactants = Mass of products

Example:
When 12 g of carbon reacts with 32 g of oxygen to form carbon dioxide:
C + O₂ → CO₂
12 g + 32 g = 44 g (mass of CO₂ formed)

🧠 Is law of conservation of mass valid when particles of light break from the packet of quanta?
No, the classical Law of Conservation of Mass stated by Lavoisier is not strictly valid in reactions involving the emission or absorption of light (photons) at the atomic, nuclear, or particle physics level.
When particles of light break from the packet of quanta (i.e., photons are emitted), the emitting system does lose an extremely small amount of mass — the classical Law of Conservation of Mass is violated at that level of precision, but the broader Law of Conservation of Mass–Energy remains perfectly valid.

2. Law of Definite Proportions (Law of Constant Composition)

Statement: A chemical compound always contains the same elements in the same fixed proportion by mass, regardless of the source or method of preparation.

Proposed by: Joseph Proust (1799)

Example:
In water (H₂O), the mass ratio of hydrogen to oxygen is always 1:8 (or 2:16 by atoms).
Whether obtained from rain, river, or synthesized in the lab, water always has 11.11% hydrogen and 88.89% oxygen by mass.

🧠 Can we apply the law of definite proportion in case of nonstoichiometric compounds and polymers?
No, The Law of Definite Proportions is valid only for compounds that have a fixed, invariable atomic ratio in every sample, regardless of how they are prepared.
Therefore:
Non-stoichiometric compounds → do not obey the law (composition is variable).
Most polymers → do not obey the law (variable chain length and sometimes monomer ratio).

3. Law of Multiple Proportions

Statement: When 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 a ratio of small whole numbers.

Proposed by: John Dalton (1803)

Example:
Carbon forms two oxides:
• CO (carbon monoxide): 12 g carbon + 16 g oxygen
• CO₂ (carbon dioxide): 12 g carbon + 32 g oxygen
For fixed 12 g of carbon, oxygen masses are 16 g and 32 g → ratio 16:32 = 1:2

4. Law of Reciprocal Proportions (Law of Equivalent Weights)

Statement: When two elements separately combine with a fixed mass of a third element, the ratio in which they do so is the same as or a simple multiple of the ratio in which they combine with each other.

Proposed by: Jeremias Richter (1792)

Example:
Hydrogen combines with oxygen to form water (H:O = 1:8)
Hydrogen combines with chlorine to form HCl (H:Cl = 1:35.5)
Oxygen and chlorine should combine in the ratio 8:35.5 or 16:71 (as in compounds like Cl₂O)

5. Gay-Lussac's Law of Gaseous Volumes

Statement: When gases react, they do so in volumes that bear a simple ratio to each other and to the volumes of the gaseous products, provided temperature and pressure remain constant.

Proposed by: Joseph Louis Gay-Lussac (1808)

Example:
2 volumes of hydrogen + 1 volume of oxygen → 2 volumes of water vapor
Volume ratio: 2:1:2

6. Avogadro's Law (Avogadro's Hypothesis)

Statement: Equal volumes of all gases, under the same conditions of temperature and pressure, contain an equal number of molecules (or moles).

Proposed by: Amedeo Avogadro (1811)

       

V ∝ n     or     V₁ / n₁ = V₂ / n₂

Example:
Based on Gay-Lussac's ratio (2 volumes H₂ + 1 volume O₂ → 2 volumes H₂O),
Avogadro's Law explains that this corresponds to the molecular ratio:
2 molecules H₂ + 1 molecule O₂ → 2 molecules H₂O

        

🧠 What is the consequence of Avogadro's Law?
This law led directly to the concept of Molar Volume. At Standard Temperature and Pressure (STP), one mole of any ideal gas occupies a fixed volume of approximately 22.4 liters.

🧠 A container of 5 liter consists CO₂ gas. Another container of half the volume consists O₂ at same condition. Would it follow Avogadro's law?
Yes, the given situation perfectly follows Avogadro's law.
Given:
Volume of CO₂ container (V₁) = 5 L
Volume of O₂ container (V₁) = 2.5 L
Temperature (T) and Pressure (P) are the same for both containers.
According to Avogadro’s law:
Number of molecules ∝ Volume (at constant T and P)
∴ Ratio of number of molecules = V₁ : V₂ = 5 : 2.5 = 2 : 1
That means the 5 L container of CO₂ has twice the number of molecules as the 2.5 L container of O₂, which is exactly what Avogadro's law predicts.
Conclusion: The situation completely obeys Avogadro’s law. The nature of gas (CO₂ or O₂) does not matter — only volume, temperature and pressure matter.

Note: These six laws provided strong experimental evidence for Dalton's Atomic Theory and laid the foundation for modern stoichiometry.

Test Your Knowledge

MCQs Asked in NEET, IIT-JEE, IIMS, AIPMT

JEE Main 2020 Equivalent

Q1: Which of the following pairs of compounds correctly illustrates the Law of Multiple Proportions?

1. Water (H₂O) and Hydrogen Peroxide (H₂O₂)
2. Sodium Chloride (NaCl) and Potassium Chloride (KCl)
3. Sulphur Dioxide (SO₂) and Sulphur Trioxide (SO₃)
4. Both A and C

Correct Answer: D

The Law of Multiple Proportions applies when two elements combine to form two or more different compounds.A. In H₂O and H₂O₂, Hydrogen and Oxygen combine.
For a fixed mass of H (2 units), the mass of O is 16 and 32, respectively.
Ratio = 16:32 = 1:2.
In SO₂ and SO₃, Sulphur and Oxygen combine.
For a fixed mass of S (32 units), the mass of O is 32 and 48, respectively.
Ratio = 32:48 = 2:3.
Since both pairs obey the law, D is the correct choice.

NEET 2022 Equivalent

Q2: A sample of pure Barium Chloride (BaCl₂) obtained from different sources was analyzed. It was found that 5.0 g of the first sample contained 2.43 g of Barium, and 10.0 g of the second sample contained 4.86 g of Barium. This data supports which of the following laws?

1. Law of Conservation of Mass
2. Law of Multiple Proportions
3. Law of Definite Proportions
4. Law of Reciprocal Proportions

Correct Answer: C

The Law of Definite Proportions states that a chemical compound always contains the same elements in the same fixed proportion by mass, regardless of the source.
Sample 1: Mass % of Barium = (2.43 g / 5.0 g) x 100% = 48.6
Sample 2: Mass % of Barium = (4.86 g / 10.0 g) x 100% = 48.6
Since the percentage composition is the same for both samples, the law is supported.

AIIMS 2017 Equivalent

Q3: 10.0 g of CaCO₃ on heating gives 4.4 g of CO₂ and 5.6 g of CaO. This observation is in agreement with:

1. Law of Definite Proportions
2. Law of Conservation of Mass
3. Law of Multiple Proportions
4. Avogadro's Law

Correct Answer: B

The chemical reaction is CaCO₃ → CaO + CO₂.
Mass of Reactant (CaCO₃) = 10.0g.
Total Mass of Products (CaO + CO₂) = 5.6 g + 4.4 g = 10.0 g.
Since Mass of Reactants = Mass of Products
The data supports the Law of Conservation of Mass.

JEE Advanced 2019 Concept

Q4: Which of the following compounds is the most likely exception to the Law of Definite Proportions?

1. Glucose
2. Water
3. Iron Oxide
4. Ammonia

   

Correct Answer: C

The Law of Definite Proportions is only valid for stoichiometric compounds. Fe_0.93O is a non-stoichiometric compound (a Berthollide compound) which has a variable composition due to crystal lattice defects (missing Fe atoms). Hence, its composition is not fixed, violating the law.

NEET/JEE Gas Law Application

Q5: 1 volume of Nitrogen gas reacts with 3 volumes of Hydrogen gas to produce 2 volumes of Ammonia gas. If all volumes are measured at the same temperature and pressure, this observation demonstrates:

1. Law of Conservation of Mass
2. Law of Definite Proportions
3. Gay-Lussac's Law of Gaseous Volumes
4. Law of Multiple Proportions

   

Correct Answer: C

The reaction is N₂(g) + 3H₂(g) → 2NH₃(g).
The volumes of reactants (N₂ and H₂) and the product (NH₃) are in a simple whole number ratio of 1:3:2.

JEE Main 2021 Equivalent (Avogadro's Law)

Q6: Under identical conditions of temperature and pressure, the ratio of molecules in 5.0 L of O₂ to 10.0 L of SO₂ would be:

1. 1:1
2. 1:2
3. 2:1
4. 4:1

Correct Answer: B

Avogadro's Law states that under identical conditions of temperature (T) and pressure (P), the volume (V) of a gas is directly proportional to the number of moles/molecules (n). Thus, the ratio of molecules is equal to the ratio of their volumes:
Ratio of molecules (O₂ : SO₂) = Ratio of volumes (V_O2 : V_SO2)
Ratio = 5.0 L : 10.0 L = 1:2

NEET 2020

Q7: The law of conservation of mass was explained by:

1. Law of definite proportions
2. Dalton's atomic theory
3. Avogadro's law
4. Gay-Lussac's law

Correct Answer: B

Dalton explained the law of conservation of mass by stating that atoms are neither created nor destroyed in a chemical reaction.

JEE Main 2020

Q8: In water (H₂O), hydrogen and oxygen are always present in the ratio 1 : 8 by mass. This illustrates the law of:

1. Conservation of mass
2. Constant composition
3. Multiple proportions
4. Reciprocal proportions

Correct Answer: B

Law of constant (definite) composition states that a compound always contains the same elements in a fixed ratio by mass.

NEET 2023

Q9: The law of multiple proportions is illustrated by:

1. H₂O and D₂O
2. CO and CO₂
3. NaCl and KCl
4. NH₃ and PH₃

Correct Answer: B

In CO and CO₂, the masses of oxygen that combine with a fixed mass of carbon are in the ratio 1:2, illustrating the law of multiple proportions.

AIPMT 2014

Q10: Equal masses of H₂, O₂ and methane have been taken in a container of volumes V at temperature 27°C in identical conditions. The ratio of the volumes of gases H₂:O₂:methane would be:

1. 8 : 16 : 1
2. 16 : 8 : 1
3. 16 : 1 : 2
4. 8 : 1 : 2

Correct Answer: C

16 : 1 : 2