Activity 3.2 NCERT Class 9 Science Atoms and Molecules

Activity 3.2 NCERT Class 9 Science Chapter 3 Atoms and Molecules

Activity 3.2 NCERT Class 9 ScienceBrief Procedure:

Activity 3.1 asks us to find the ratio of the number of atoms in various compounds. Answer: Water (H2O): 2:1 Ammonia (NH3): 1:3 Carbon dioxide (CO2): 1:2


A compound or molecule reacts with another and forms a different compound. Here the proportion in which they react always remain constant. For example, in real experiments, we always see that 2 gm of hydrogen always react with 16gm of oxygen and forms a water molecule. Thus water has always molecules in the ratio of 1:8. We call this dogma as the “law of definite proportion”.

Practical implication:

If we know the chemical formula of a compound, then the ratio of the number of atoms is simply the numbers in the formula. E.g. Atomic ratio of hydrogen and oxygen in H2O is 2:1.

If we are given the ratio of masses of a substance, then we divide it by their atomic masses. The number we get is the ratio in the number of the atoms.


The mass ratio of H2O is 1:8.

Number of H atoms = Mass ratio of Hydrogen in water/atomic weight of hydrogen = 1:1 = 1

Similarly, the number of O atoms = 8:16 = 1/2

Simplest ratio: We calculate the simplest ratio by multiplying the no with the highest common factor (HCF).

So, for hydrogen ratio of atoms become 1 X 2 = 2

For oxygen = 1/2 X 2 =1.

Similarly, ammonia NH3 has a weight ratio as  14/3, and their respective atomic weights are 14 and 3.

Here, the ratio of the atoms will be 1:3.

Similarly, carbon dioxide has a weight ratio of 3:8.

The number of carbon atoms in the compound = 3/12 = 1/4.

The number of oxygen atoms in CO2 = 8:16 = 1/2.

So the simplest ratio is 1:2.


If we know the number of reactants and amount of product formed in the chemical reaction, we can calculate their ratio of weights and the chemical formula.


See also: Experiment to demonstrate that under a chemical reaction mass remain always constant. Law of conservation of mass; Activity 3.1.

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