Class 9 Science Chapter 3 – Atoms and Molecules Intext Questions and Answers | NCERT Solutions

Understanding the fundamental concepts of atoms and molecules is essential for building a strong base in Chemistry. In this chapter, you learn how matter is made up of tiny particles and how these particles combine to form different substances.

To help students prepare better, we have provided NCERT Class 9 Science – Atoms and Molecules Intext Questions and Answers in a clear and easy-to-understand format. These solutions will help you revise important concepts, score well in exams, and strengthen your overall understanding of the chapter.

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Ch:-3 – ATOMS AND MOLECULES

Intext Solution

Ques 1:- 7. In a reaction 5.3 g of sodium carbonate reacted with 6 g of ethanoic acid . The products were 2.2 g of carbon dioxide, 0.9 g water and 8.2 g of sodium ethanoate.Show that these observations are in agreement with the law of conservation of mass.

Sodium carbonate + ethanoic acid → sodium ethanoate + Carbon dioxide + water.

Ans :- In the given reaction, sodium carbonate reacts with ethanoic acid to produce sodium ethanoate, carbon dioxide and water.

Now,
Mass of sodium carbonate: 5.3 g
Mass of ethanoic acid: 6 g

Total mass of reactant = (5.3 + 6) g = 11.3 g

Also,
Mass of sodium ethanoate = 8.2 g
Mass of carbon dioxide = 2.2 g
Mass of water = 0.9 g

Total mass of Product = (8.2 + 2.2 + 0.9) g = 11.3 g

Therefore Total mass of reactant = Total mass of Product

Hence the given observations are in agreement with the law of conservation of mass.

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Ques 2:- Hydrogen and oxygen combine in the ratio of 1 : 8 by mass to form water.What mass of oxygen gas would be required to react completely with 3 g of hydrogen gas ?

Ans :- Given Hydrogen and oxygen combine in the ratio of 1 : 8 by mass to form water (H₂O).

Its mean that 1 g of hydrogen gas will reacts completely with 8 g of Oxygen.

∴ 3 g of hydrogen gas will reacts completely with 8 × 3 = 24 g of Oxygen.

Hence 24 g of oxygen gas would be required to react completely with 3 g of hydrogen gas.

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Ques 3:- Which postulate of Dalton’s atomic theory is the result of the law of conservation of mass ?

Ans:-The postulate of Dalton’s atomic theory which is a result of the law of conservation of mass is "Atoms are indivisible particles, which can neither be created nor destroyed in a chemical reaction".

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Ques 4:- Which postulate of Dalton’s atomic theory can explain the law of definite proportions ?

Ans:- The postulate of Dalton’s atomic theory which can explain the law of definite proportion is "The relative number and kind of atoms in a given compound remain constant ".

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Ques 1:- Define atomic mass unit.

Ans:- The atomic mass unit is defined as the mass equal to 1/12 th the mass of ¹²₆C atom. It is written as "U".

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Ques 2:- Why is it not possible to see an atom with naked eyes?

Ans:- The size of an atom is so small that it is not possible to see it with naked eyes. Also, atom of an element does not exist independently.

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Ques 1:- Write down the formula of

Ans:- (i) Sodium oxide
(ii) Aluminium chloride
(iii) Sodium sulphide
(iv) Magnesium hydroxide.

Ans (i) :- Formula of Sodium oxide

Formula: - Na₂O

Ans (ii) :- Formula of Aluminium chloride

Formula: - AlCl₃

Ans (iii) :- Formula of Sodium sulphide

Formula: - Na₂S

Ans (iv) :- Formula of Magnesium hydroxide

Formula: - Mg(OH)₂

Ques 2:- Write down the name of compounds represented by the following formulae :

Al₂(SO₄)₃ , CaCl₂ , K₂SO₄ , CaCO₃

Ans:-

Chemical Formula	Name of Compound
(i) Al2(SO4)3	Aluminium sulphate
(ii) CaCl2	Calcium chloride
(iii) K2SO4	Potassium sulphate
(iv) KNO3	Potassium nitrate
(v) CaCO3	Calcium carbonate

Ques 3:- What is meant by the term chemical formula ?

Ans:- The chemical formula of a compound is a symbolic representation of its composition.

For example chemical formula of carbon dioxide is CO₂, we come to know that one carbon atom and two oxygen atoms are chemically bonded together to form one molecule of the compound, carbon dioxide.

Ques 4:- How many atoms are present in a

(i) H₂S molecule , and
(ii) PO₄ ion ?

Ans :- (i) In a H₂S molecule, three atoms are present;
two of hydrogen and one of sulphur.

(ii) In a PO₄ ion, five atoms are present;
one of phosphorus and four of oxygen.

Page 35

Ques 1:- Calculate the molecular masses of

H₂, O₂, Cl₂, CO₂, CH₄, C₂H₆, C₂H₄, NH₃, CH₃OH,

(i) Molecular mass of H₂ = 2 × Atomic mass of H

= 2 × 1u
= 2u

(ii) Molecular mass of O₂ = 2 × Atomic mass of O

= 2 × 16u
= 32u

(iii) Molecular mass of Cl₂ = 2 × Atomic mass of Cl

= 2 × 35.5u
= 71u

(iv) Molecular mass of CO₂ = 2 × Atomic mass of C + 2 x Atomic mass of O

= 12u + 2 x 16u
= 12u + 32u
= 44u

(v) Molecular mass of CH₄ = Atomic mass of C + 4 x Atomic mass of H

= 12u + 4 x 1u
=12u + 4u
= 16u

(vi) Molecular mass of C₂H₆ = 2 x Atomic mass of C + 6 x Atomic mass of H

= 2 x 12u + 6 x 1u
= 24u + 6u
= 30u

(vii) Molecular mass of C₂H₄ = 2 x Atomic mass of C + 4 x Atomic mass of H

= 2 x 12u + 4 x 1u
= 24u + 4u
= 28u

(viii) Molecular mass of NH₃ = Atomic mass of N + 3 x Atomic mass of H

= 14u + 3 x 1u
= 14u + 3u
= 17u

(ix) Molecular mass of CH₃OH = Atomic mass of C + 3 x Atomic mass of H + Atomic mass of O + Atomic mass of H

= 12u + 3 x 1u + 16u + 1u
= 12u + 3u + 16u + 1u
= 32u

Ques 2:- Calculate the formula unit masses of :

ZnO₂ , Na₂O , K₂CO₃
given atomic masses of Zn = 65 u , Na = 23 u , K = 39 u , C = 12 u , and O = 16 u .

Ans:-

(i) Atomic mass of Zn = 65u
Atomic mass of O = 16u

Formula unit mass of ZnO = Atomic mass of Zn + Atomic mass of O

= 65 + 16

Formula unit mass of ZnO = 81 u

(ii) Atomic mass of Na = 23u
Atomic mass of O = 16u

Formula unit mass of Na₂O = 2 x Atomic mass of Na + Atomic mass of O

= 2 x 23 + 16 =62

Formula unit mass of Na₂O = 62u

(iii) Atomic mass of K= 39u
Atomic mass of C = 12u
Atomic mass of O= 16u

Formula unit mass of K₂CO₃ = 2 x Atomic mass of K + Atomic mass of C + 3 x Atomic mass of O

= 2 x 39 + 12 + 3 x 16 = 138

Formula unit mass of K₂CO₃ = 138 u

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Conclusion

The chapter “Atoms and Molecules” helps us understand how matter is made up of tiny particles and how these particles combine in fixed ratios to form different substances.By solving the intext questions, students become familiar with key concepts such as atomic mass,molecular mass, chemical formulas, laws of chemical combination, and the idea of moles.These concepts form the foundation of chemistry and are essential for understanding reactions,calculations, and the composition of various chemical compounds.