Laws of Chemical Combination for Elements and Compounds - Meaning, Definition, Examples, FAQs

The laws of chemical combination

Whenever substances react, they do so according to certain laws. These laws are called the laws of chemical combinations. These basic laws are five in number and formed the basis of Dalton's Atomic Theory.

Law of Conservation of Mass :

It states that Matter can neither be created nor destroyed. This law formed the basis for several later developments in chemistry. In fact, this was the result of the exact measurement of masses of reactants and products, and carefully planned experiments performed by Lavoisier.

Law of Definite Proportions :

This law was given by, a French chemist, Joseph Proust. He stated that a given compound always contains exactly the same proportion of elements by weight.

Proust wprked with two samples of cupric carbonate - one of which was of natural origin and theother was synthetic one. He found that the composition of elements present in it was same for both the samples as shown below:

Thus, irrespective of the source, a chemical compound always contains the elements in fixed ratio by mass.

Law of Multiple Proportions :

This law was proposed by Dalton in 1803. According to this law, if two elements can 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.

For example, carbon and oxygen combines with each other to form carbon monoxide (CO) and carbon dioxide (CO₂).

In CO : 12 parts by mass of carbon combine with 16 part by mass of oxygen.

In CO₂ : 12 parts by mass of carbon combine with 32 parts by mass of oxygen.

Ratio of masses of oxygen which combine with fixed mass of carbon in these compounds is 16:32 or 1:2, which is a simple whole number ratio.

Gay Lussac’s Law of Gaseous Volumes :

This law was given by Gay Lussac in 1808. He observed that 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.

This law may be illustrated by the following example:

It has been found that one volume of nitrogen combines with three volumes of hydrogen to form two volumes of ammonia gas.

$\mathrm{N}_2+3 \mathrm{H}_2 \rightleftharpoons 2 \mathrm{NH}_3$

Therefore, ratio by volumes of nitrogen, hydrogen and ammonia is 1:3:2, which is a simple whole number ratio.

Avogadro Law :

• According to this law, "Under similar conditions of temperature and pressure, the equal volume of gases the equal number of molecules. "

It means 10 ml of H₂, O₂, N₂ or a mixture of gases have the same number of molecules.

It is used in:

(i) The deriving molecular formula of a gas

(ii) Determining atomicity of a gas

(iii) Deriving a relation

molecular mass = 2 x vapour density

M=2 X VD

(iv) Deriving the gram molecular volume

• Avogadro number (N_o or N_A) = 6.023 x 10²³
• Avogadro number of gas molecules occupy 22.4 litre or 22400 ml or cm³ volume at STP
• The number of molecules in 1 cm³ of a gas at STP is equal to Loschmidt number, that is, 2.68 x 10¹⁹
• The reciprocal of Avogadro number is known as Avogram.

Related Topics,

• Law Constant Proportion
• Balancing Chemical Equations
• Mole Concept Basics
• Difference Between Physical and Chemical Change
• Difference Between Compound and Mixture

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Recommended topic video on (Laws of Chemical Combination for elements)

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Some Solved Examples

Example.1

The ratio of masses of oxygen and nitrogen in a particular gaseous mixture is 1:4. The ratio of a number of their molecule is :

1) 1 : 4

2) 7 : 32

3) 1 : 8

4) 3 : 16

Solution

We know

Mole = mass/molar mass

Avogadro's number is defined as the number of elementary particles (molecules, atoms, compounds, etc.) per mole of a substance. It is equal to 6.022×10²³ mol^-1 and is expressed as the symbol N_A.

Now let the ratio be m, therefore, the mass of O₂ is m and the mass of N₂ is 4m.
Therefore, moles of O₂ = m/32 and moles of N₂ = 4m/28

Thus, $\frac{\text { molecules }_{\mathrm{O}_2}}{\mathrm{molecules}_{\mathrm{N}_2}}=\frac{\frac{\mathrm{m}}{32} \times \mathrm{N}_{\mathrm{A}}}{\frac{4 \mathrm{~m}}{28} \times \mathrm{N}_{\mathrm{A}}}=\frac{7}{32}$

Hence, the answer is the option (2).

Example. 2

What is the ratio of masses of oxygen that combine with a fixed mass of Hydrogen in water and Hydrogen Peroxide?

1) 1 : 2

2) 2 : 1

3) 2 : 3

4) 3 : 2

Solution

As we learnt in

$\mathrm{H}_2+\frac{1}{2} \mathrm{O}_2 \rightarrow \mathrm{H}_2 \mathrm{O}$

2g 16g 18g

$\mathrm{H}_2+\mathrm{O}_2 \rightarrow \mathrm{H}_2 \mathrm{O}_2$

2g 32g 34g

One mole of water has 2g hydrogen & 16g oxygen.

Similarly, one mole of hydrogen peroxide has 2g of hydrogen and 32 g of oxygen

Therefore, masses of oxygen combined with a fixed mass of hydrogen in water and hydrogen peroxide have a ratio of 16:32 = 1:2

Hence, the answer is the option (1).

Example .3

What is the ratio of the masses of oxygen that combine with a fixed value of nitrogen in the compounds nitrogen monoxide and nitrogen dioxide?

1) 2 : 3

2) 2 : 1

3) 3 : 2

4) 1 : 2

Solution

Let's take NO first, the chemical reaction follows:

$N_2+O_2 \rightarrow N O$

$\frac{\text { Mass of } N}{\text { mass of } O}=\frac{14}{16}=1: 1.143$

in NO₂,

$2 \mathrm{NO}+\mathrm{O}_2 \rightarrow 2 \mathrm{NO}_2$

there are 2 N and 4 O in the reaction,

$\frac{\text { Mass of } N}{\text { mass of } O}=\frac{28}{64}=1: 2.286$

$\therefore \frac{\text { Mass of } \mathrm{O} \text { in } \mathrm{NO}}{\text { mass of } \mathrm{O} \text { in } \mathrm{NO}_2}=\frac{1.143}{2.286}=1: 2$

Hence, the answer is the option (4).

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Summary

The laws of chemical combination expresses how elements form compounds and directed our understanding of chemical reactions. The conservation of mass make sure that reactant mass equals product mass in a closed system. The law of definite proportions states that a compound always contains the same elements in fixed mass ratios, no matter how or where it's made. The law of multiple proportions shows that when two elements form different compounds, the ratios of one element combining with a fixed mass of the other are simple whole numbers ratio. These principles form the foundation of stoichiometry and expected chemistry.