Atoms and Molecules

Laws of Chemical Combination

With the development of more scientific ideas on elements, compounds and mixtures, people began to probe into how and why substances react. French chemist A. Lavoisier laid the foundation to the scientific investigation of matter by describing that substances react by following certain laws. These laws are called the laws of chemical combination. These later on formed the basis of Dalton's Atomic Theory of Matter.

Law of Conservation of Mass

According to this law, during any physical or chemical change, the total mass of the products remains equal to the total mass of the reactants.

Law of conservation of mass is also known as 'Law of indestructibility of matter'.

Example 1:

If 10 grams of CaCO₃ on heating gave 4.4g of CO₂ and 5.6g of CaO, show that these observations are in agreement with the law of conservation of mass.

Solution:

Mass of the reactants = 10gMass of the products = 4.4 + 5.6g = 10gSince the mass of the reactants is equal to the mass of the products, the observations are in agreement with the law of conservation of mass.

Law of Constant Proportion

Another French chemist, Joseph Proust, stated this law as 'A chemical compound always contains same elements combined together in the same proportion by mass'.

For example, pure water obtained from different sources such as river, well, spring, sea, etc., always contains hydrogen and oxygen together in the ratio of 1: 8 by mass. Similarly, carbon dioxide CO₂ can be obtained by different methods such as,

• Burning of carbon
• Heating of lime stone
• By action of dilute HCl on marble pieces

But the different samples of CO₂ always contain carbon and oxygen in the ratio of 3: 8.

Example 2 :

When 1.375g of cupric oxide was reduced on heating in a current of hydrogen, the weight of copper that remained was 1.098g. In another experiment 1.179g of copper was dissolved in nitric acid and resulting copper nitrate converted into cupric oxide by ignition. The weight of cupric oxide formed was 1.476g. Show that these results illustrate the law of constant proportion.

Solution:

First experiment

Copper oxide = 1.375g

Copper left = 1.098g

Oxygen present = 1.375 - 1.098 = 0.277g

Second Experiment

Copper taken = 1.179g

Copper oxide formed = 1.476g

Oxygen present = 1.476 - 1.179 = 0.297g

Percentage of oxygen is approximately the same in both the above cases. So the law of constant composition is illustrated.

Law of Multiple Proportions

John Dalton (1803) stated this law as 'when two elements combine with each other to form two or more than two compounds, the mass of the element which combine with the fixed mass of the other bears a simple whole number ratio'.

Examples 3:

Carbon monoxide (CO): 12 parts by mass of carbon combines with 16 parts by mass of oxygen.

Carbon dioxide (CO2): 12 parts by mass of carbon combines with 32 parts by mass of oxygen.

Ratio of the masses of oxygen that combines with a fixed mass of carbon (12 parts) 16: 32 or 1: 2

For example, hydrogen and oxygen are known to form 2 compounds. The hydrogen content in one is 5.93% while in the other it is 11.2%. Show that this data illustrates the law of multiple proportions.

Solution:

In the first compound hydrogen = 5.93%

Oxygen = (100 -5.93) = 94.07%

In the second compound hydrogen = 11.2%

Oxygen = (100 -11.2) = 88.88%

Ratio of the masses of oxygen that combine with fixed mass of hydrogen 15.86: 7.9 or 2:1

The ratio illustrates the law of multiple proportions.