Atoms and Molecules| Worksheet Solutions| Class 9

Worksheet solutions for chapter: Atoms and Molecules for class 9 are provided below.

(1) What are the laws of chemical combination? State.


These laws are established by the chemist Antonie Lavoisier and Joseph L. Proust.

(i)Law of conservation of the mass states that total mass of all reactants are equal to total mass of the products in a chemical reaction.

(ii) Law of constant proportion states that in a chemical substance the elements are always present in definite proportions by mass.


(2) Write difference between atomic mass and molecular mass.


SN Atomic Mass Molecular mass
1. Atomic Mass of an element is the average mass of the atoms of element.


The sum of the atomic masses of the all atoms in a molecule of the substance is called molecular mass
2. Eg: Atomic mass of the oxygen is 16 u. Eg:- Calculate the relative mass (Molecular mass) of the H2.

Ans- Atomic mass of the hydrogen = 1 u

Molecular mass of the H2 = 2(Atomic mass of the hydrogen)

= 2 u

Hence, the relative mass of the hydrogen molecule is 2 u.



(3)What is the atomicity? Give an example.


The number of atoms constituting a molecule is known as its atomicity.


2H2  + O2 → 2H2O

(4)What is ion?


An ion is a single charged atom or group of atoms (Molecules) that have a net charge on them.

Types of ions

(i) A negative charged ion is called anion.

Eg:- A chlorine atom gains one ion to become anion (Cl)

(ii) A positive charged ion is called cation.

Eg:- A sodium atom losses one ion to become cation (Na+)

(iii) When two or more ions considered as single unit, are called polyatomic ion.

Eg:- Hydroxide ion consists one ion of oxygen and one ion of hydrogen.


(5)Write the chemical formula of H2O and H2S.



(6) What is molecular mass? Calculate the molecular mass of H2O2.


The sum of the atomic masses of the all atoms in a molecule of the substance is called molecular mass.

The unit of molecular mass is “u”.


(7)What is mole?


One Mole of any atoms, molecules, ions or particles is that quantity in number having a mass equal to its atomic or molecular mass in grams.

1 mole = 6.022 × 1023 number of particles or ions in number

Mass of 1 mole = relative mass of particles or ions in grams


(8) Convert 12 g of nitrogen in to mole.


(9)What is the mass of 0.5 moles of oxygen atoms?


(10) Calculate mass of the 6.022 ×1023 number of N2 molecules (mass from number).


Helping Topics

Atoms and Molecules

NCERT Solutions Class 9