Laws of Chemical Combinations

During the seven-teeth century, the scientists observed that there existed certain regularities and definite pattern in the mass-volume, mass-mass and volume-volume relationships between the reactants and products.

These similarities were grouped in the form of five laws. These laws deal with the composition of substances by mass or by volume, and are called the Laws of Chemical Combination or Laws of Stoichiometry.

Law of conservation of Mass

This law was put forward by Lomonosov in 1736. It states that

Definition 1
The total mass of matter in any chemical or physical change remains constant though the matter may change its form.

Definition 2
In any chemical reaction, the total mass of system before and after reaction is the same. It may however undergo a change in its physical form.

Definition 3
Matter is neither created nor destroyed as a result of any chemical or physical change.

Let me explain this to you using an example of a chemical reaction.

Let’s say that we’re performing a chemical reaction in which two reactants A, B react together to form two products C, D.

So this reaction’s equation can be written as A + B → C + D

Let’s say if in this chemical reaction
a grams of A
b grams of B
to product
c grams of C
d grams of D

Total mass of reactants = a + b grams
Total mass of products = c + d grams

As per this law, Total mass of reactants should be equal to Total mass of products
a + b = c + d

Experimental Demonstration of The Law of Conservation of Mass

Hans Heinrich Landolt verified law of conservation of mass by performing a simple experiment described below.

In this experiment, a H-Shaped glass apparatus was used. In one of its limbs, a solution of sodium chloride (NaCl) was filled, while in the second limb a solution of silver nitrate (AgNO3) was filled.

Two tubes attached with each other making U-shaped Tube, one tube containing silver nitrate and other containing sodium chloride both in aqueous form - apparatus for Landolt's experiment to verify law of conservation of mass

Then he measured weight of each of tube and noted it. After he shook the apparatus so that both of compounds can react together, after that he put apparatus on rest for some time so that reaction is fully complete.

AgNO3 (aq) + NaCl (aq) AgCl (s) + NaNO3 (aq)

Then again he measured weight of each tube and found that total wight of apparatus(both tubes together) before reaction was same as it was after reaction. Which means despite both of compounds reacting with each other and forming new products, total weight remained the same.

This verified the law of conservation of mass.

Law of Definite Proportions or Law of Constant Composition

This law was stated by Louis Proust in 1799. It states that

A chemical compound always consists of the same elements combines together in the same ratio, irrespective of the method of preparation or the source from where it is taken.

For example
Water (H2O) is always found to contain only two elements hydrogen and oxygen combines in a definite mass ration of 1:8 irrespective of its source.

Similarly, sodium chloride (NaCl) from whatever source will always contain 39.32% sodium and 60.68% of chlorine by mass.

Carbon dioxide is also found to contain C and O in the mass ratio 12:32 or 3:8 no matter what source its extracted from.

Example showing verifying of Law of Constant Composition

Numerical Example showing that Law of Constant Composition is true

Limitations of Law of Constant Composition

1. When a compound is obtained by using different isotopes of the combining elements. For example – when H2O is formed from 11H and 168O the mass ration between hydrogen and oxygen is 2:16 or 1:8 whereas when water is formed from 21H (deuterium – an isotope of hydrogen) and 168O the mass ration between hydrogen and oxygen is 4:16 or 1:4.

Thus different isotopes of the same element give different mass ratios between the combining elements.

2. The law of constant composition does not hold good for non-stoichiometric compounds. This is because, the non-stoichiometric compounds do not have fixed composition.

Law of Multiple Proportions

This law was enunciated by John Dalton in 1803. It states that

When two elements combine to form two or more compounds, then the different masses of one element which combine with a fixed mass of the other, bear a simple ratio to one another.

Example explaining Law of Multiple Proportions

Copper reacts with oxygen to give cuprous oxide (Cu2O) and cupric oxide (CuO). Nitrogen reacts with oxygen to give five oxides – N2O (nitrous oxide), NO (nitric oxide), N2O3 (dinitrogen trioxide), NO2 (nitrogen dioxide) and N2O5 (nitrogen pentaoxide).

Let’s see mass composition of each of these nitrogen oxides.

of nitrogen
of Nitrogen (g)
of Oxygen (g)
Mass of Oxygen
for 14 g of nitrogen
N2O3 284824

So with 14 g of nitrogen 8:16:24:32:40 grams of nitrogen reacts, simplifying this we get 1:2:3:4:5 which is a simple ratio.

Thus when two elements combine to form two or more compounds, then the different masses of one element which combine with a fixed mass of the other, bear a simple ratio to one another.

Law of Reciprocal Proportions or Law of Equivalent Proportions

This law was put forward by Jeremias Ritcher in 1792. It states that

If three elements A, B and C mutually combine to give binary compounds AB, BC and CA then the ratio of masses of B and C when they combine together, is a simple multiple of the mass ratio of B and C in which they combine separately with a fixed amount of A.

For example
Copper reacts with oxygen and sulphur to give copper oxide and copper sulphide respectively. Sulphur and Oxygen also react with each other to give sulphur oxide.


Cu : S = 63.5 : 32
Cu : O = 63.5 : 16

Therefore S : O = 32 : 16 = 2 : 1

In SO2
S : O = 32 : 32 = 1 : 1

Thus ratio of S : O from CuS, CuO and SO2 = 2 : 1 : 1 : 1 = 2 : 1 which is a simple multiple ratio

Gay-Lussac’s Law of Gaseous Volumes

This law was put forward by the French chemist, Gay-Lussac, this law established an interesting volume-volume relationship between volumes of reactants and products.
This law states that

Under constant temperature and pressure, the volumes of the gaseous reactants bear a simple ratio between themselves and also with the volumes of products, if gaseous.

For example

In chemical reaction of hydrogen and chlorine to form hydrogen chloride
H2 (g) + Cl2 (g) → 2 HCl (g)

Ratio of volumes of hydrogen, chlorine and hydrogen chloride is 1:1:2 which is a simple ratio.

Similarly in chemical reaction of hydrogen and oxygen to form water vapours
2 H2 (g) + O2 (g) → 2 H2O (g)

Ratios of volumes of hydrogen, oxygen and water vapours is 2:1:2 which is a simple ratio

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