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CBSE Class 9 Science Important Extra Questions from Chapter 3 - Atoms and Molecules (with Answers)

Gurmeet Kaur

Check NCERT based extra questions for CBSE Class 9 Science Chapter 3 - Atoms and Molecules. Accurate answers have been provided for all the questions. All the questions are based on important concepts. These extra questions are important for self-assessment and prepare effectively for your exams. You can read and download all the questions and answers to use them for your exam preparations.

CBSE Class 9 Science Extra Questions for Chapter 3 - Atoms and Molecules:

1. State:

(a) Law of Conservation of mass

(b) Law of constant proportion


(a) It states that mass can neither be created nor destroyed in a chemical reaction.

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

2. What do you mean by atomicity? What is the atomicity of ozone?


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

Ozone (O3)​ contains 3 oxygen atoms. So its atomicity is 3.

3. Calculate the mass of 1 atom of Hydrogen.

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One mole of hydrogen weighs = 1 g

∴ 6.02×1023 atoms of hydrogen weigh = 1/ (6.02×1023)

∴ 1 atom of hydrogen weighs = 1.66 ×10−24 g

4. Name a non-metal which is polyatomic.


Sulphur (S) called polyatomic because each sulphur molecule (S8) is made up of 8 sulphur atoms.

5. Calculate the mass percentage of oxygen present in HNO3.


The molecular mass of HNO3 = the atomic mass of H + the atomic mass of N + 3 × the atomic mass of O = 1 + 14 + 48 = 63 g

Mass percentage of oxygen = (Mass of O in compound/ molecular mass) x 100

                                                   = (48/63) x 100 = 76.19%

Also Check:

NCERT Book for Class 9 Science

NCERT Solutions for Class 9 Science

6. What is Avogadro constant? Write its value.


The Avogadro constant is defined as the number of atoms in exactly 12 g of carbon-12.

Avogadro constant or Avogadro number = 6.022 × 1023

7. What do you mean by one mole of a substance?


The mole is the amount of substance that contains the same number of particles (atoms/ ions/ molecules/ formula units etc.) as there are atoms in exactly 12 g of carbon-12.

8. How many molecules of water are present in 36 grams of water?


Molecular mass of water (H2O) = 2(H)+1(O) = 2×1 + 1×16 = 2 + 16 = 18 gm

Thus, 1 mole of water weighs= 18 g

Or 18g of water contain 1 mole, i.e., 6.023 x 1023 molecules of water.

36 gm of water contain = (36/18) x 6.023 x 1023

= 2 x 6.023 x 1023 = 12.044 x 1023molecules of water

9. Which has more number of atoms: 100g of N2 or 100g of NH3?


NH₃ has more atoms than N₂.

10. A group of atoms carrying a fixed charge on them is called _______.


Polyatomic ion.

You may download all the above questions and answers in PDF from the link given below:

Download CBSE Class 9 Science Extra Questions for Chapter 3 - Atoms and Molecules

Students must go through the latest CBSE Syllabus for Class 9 Science so that they can prepare according to the contents prescribed by the board.

Also check:

CBSE Class 9 Science Extra Questions and Answers - All Chapters

CBSE Class 9 Science Important Questions and Answers

CBSE Class 9 Science Important MCQ (Chapter-wise)

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