NCERT Solutions For Class 9 Science Chapter- 3 Atoms and Molecules

NCERT Solutions For Class 9 Science

Chapter 3 Atoms and Molecules


NCERT Solutions Class 9 Science Chapter 3 Atoms and Molecules – Here are all the questions answers for Class 9 Science Chapter 3. If you are a student of class 9 who is using NCERT Textbook to study Science, then you should go through Chapter 3 Atoms and Molecules. After studying the lesson, if you are looking for answers of its questions. Then here is the solutions. You will get complete NCERT Solutions for Class 9 Science Chapter 3 Atoms and Molecules here.

NCERT Solutions For Class 9 Science Chapter 3 Atoms and Molecules

Topics and Sub Topics covered in Class: 9 Science Chapter: 3 Atoms and Molecules:-

  1. Atoms and Molecules
  2. Laws of Chemical Combination
  3. What is an Atom?
  4. What is a Molecule?
  5. Writing Chemical Formulae
  6. Molecular Mass and Mole Concept

These questions answers are the part of NCERT Solutions for Class 9 Science. Here we have given Class 9 NCERT Science Text book Solutions for Chapter 3 Atoms and Molecules.


IN-TEXT QUESTIONS SOLVED


NCERT Textbook for Class 9 Science – Page No. 32
Q1. 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 carbonate.

Ans:

NCERT-Solutions-For-Class-9-Science-Chapter-3-Atoms-and-Molecules-Intext-Questions-Page-32-Q1 EH

NCERT Solutions For Class 9 Science Chapter 3 Atoms and Molecules Intext Questions Page 32 Q1.1 educational hand

 

CBSE Class 9 Science Chapter 3 IN-TEXT QUESTIONS SOLVED

Class 9 Science NCERT Textbook Page 32
Q1. 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 etkanoate. Show that these observations are in agreement with the law of conservation of mass carbonate.

Ans:

Class 9 Science Chapter 3 Atoms and Molecules  Page no. 32 Q1.2

Q2. 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: Ratio of H : O by mass in water is:
Hydrogen : Oxygen —> H2O
∴ 1 : 8 = 3 : x
x = 8 x 3
x = 24 g
∴ 24 g of oxygen gas would be required to react completely with 3 g of hydrogen gas.

Q3. 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 that is the result of the law of conservation of mass is—the relative number and kinds of atoms are constant in a given compound. Atoms cannot be created nor destroyed in a chemical reaction.

Q4. Which postulate of Dalton’s atomic theory can explain the law of definite proportions?

Ans: The relative number and kinds of atoms are constant in a given compound.

Class 9 Science NCERT Textbook Page 35
Q1. Define the atomic mass unit.

Ans: One atomic mass unit is equal to one-twelfth (1/12th) of the mass of one atom of carbon-12 (C-12). The relative atomic masses of all elements have been found with respect to an atom of carbon-12.

Q2. Why is it not possible to see an atom with naked eyes?

Ans: Atom is too small to be seen with naked eyes. It is measured in nanometers.
1 m = 1000000000 nm

NCERT Textbook Questions – Page 39
Question 1. Write down the formulae of
(i) Sodium oxide
(ii) Aluminium chloride
(iii) Sodium sulphide
(iv) Magnesium hydroxide

Ans: The formula are

NCERT Solutions For Class 9 Science Chapter 3 Atoms and Molecules Intext Question Page no. 39 Q1

Q2. What is meant by the term chemical formula?

Ans: The chemical formula of the compound is a symbolic representation of its composition, e.g., chemical formula of sodium chloride is NaCl.

Q3. How many atoms are present in a
(i) H2S molecule and
(ii) P043- ion?

Ans:

(i) H2S —> Total 3 atoms are present
(ii) P043- —> Total 5 atoms are present

NCERT Textbook Questions – Page 40
Q1. Calculate the molecular masses of H2, O2, Cl2, C02, CH4, C2H2,NH3, CH3OH.

Ans: The molecular mass are following:

NCERT Solutions For Class 9 Science Chapter 3 Atoms and Molecules Intext Questions Page 40 Q1

Q2.Calculate the formula unit masses of ZnO, Na2O, K2C03, given atomic masses of Zn = 65 u, Na = 23 u, K = 39 u, C = 12 u, and O = 16 u.

Ans: The formula unit mass are following:
(i) ZnO = 65 u + 16 u = 81 u
(ii) Na2O = (23 u x 2) + 16 u = 46 u + 16 u = 62 u
(iii) K2C03 = (39 u x 2) + 12 u + 16 u x 3
= 78 u + 12 u + 48 u = 138 u

Class 9 Science NCERT Textbook Page 42
Q1. If one mole of carbon atoms weigh 12 grams, what is the mass (in grams) of 1 atom of carbon?

Ans:

NCERT Solutions For Class 9 Science Chapter 3 Atoms and Molecules Intext Questions Page no 42 Q1 educationalhand

Q2. Which has more number of atoms, 100 grams of sodium or 100 grams of iron (given atomic mass of Na = 23 u, Fe = 56 u)?

Ans:

NCERT Solutions For Class 9 Science Chapter 3 Atoms and Molecules Intext Question Page no. 42 Q2 educational hand


Questions From NCERT Textbook for Class 9 Science


Q1. A 0.24 g sample of compound of oxygen and boron was found by analysis to contain 0.096 g of boron and 0.144 g of oxygen. Calculate the percentage composition of the compound by weight.

Ans: Boron and oxygen compound —> Boron + Oxygen
0.24 g —> 0.096 g + 0.144 g

NCERT Solutions For Class 9 Science Chapter 3 Atoms and Molecules Textbook Question1

Q2. When 3.0 g of carbon is burnt in 8.00 g oxygen, 11.00 g of carbon dioxide is produced. What mass of carbon dioxide will be formed when 3.00 g of carbon is burnt in 50.00 g of oxygen? Which law of chemical combination will govern your answer?

Ans: The reaction of burning of carbon in oxygen may be written as:

NCERT Solutions For Class 9 Science Chapter 3 Atoms and Molecules Textbook Questions Q2

It shows that 12 g of carbon burns in 32 g of oxygen to form 44 g of Carbon Dioxide. Given that, 3 g of carbon reacts with 8 g of oxygen to form 11 g of carbon dioxide. It is given that 3.0 g of carbon is burnt with 8 g of oxygen to produce 11.0 g of CO2. Consequently 11.0 g of carbon dioxide will be formed when 3.0 g of C is burnt in 50 g of oxygen consuming 8 g of oxygen, leaving behind 50 – 8 = 42 g of O2.

so, The answer governs the law of constant proportion.

Q3. What are poly atomic ions? Give examples.

Ans: The ions which contain more than one atoms (same kind or may be of different kind) and behave as a single unit are called polyatomic ions example: OH, SO42-, CO32-.

Q4. Write the chemical formulae of the following:
(a) Magnesium chloride
(b) Calcium oxide
(c) Copper nitrate
(d) Aluminium chloride
(e) Calcium carbonate.

Ans:

(a) Magnesium chloride
Symbol —> Mg Cl
Change —> +2 -1
Formula —> MgCl2

(b) Calcium oxide
Symbol —> Ca O
Charge —> +2 -2
Formula —> CaO

(c) Copper nitrate
Symbol —> Cu NO
Change +2 -1
Formula -4 CU(N03)2

(d) Aluminium chloride
Symbol —> Al Cl
Change —> +3 -1
Formula —> AlCl3

(d) Calcium carbonate
Symbol —> Ca CO3
Change —> +2 -2
Formula —> CaC03

Q5. Give the names of the elements present in the following compounds:
(a) Quick lime
(b) Hydrogen bromide
(c) Baking powder
(d) Potassium sulphate.

Ans:

(a) Quick lime —> Calcium oxide
Elements —> Calcium and oxygen

(b) Hydrogen bromide
Elements —> Hydrogen and bromine

(c) Baking powder —> Sodium hydrogen carbonate
Elements —> Sodium, hydrogen, carbon and oxygen

(d) Potassium sulphate
Elements —> Potassium, sulphur and oxygen

Q6. Calculate the molar mass of the following substances.
(a) Ethyne, C2H2
(b) Sulphur molecule, S8
(c) Phosphorus molecule, P4 (Atomic mass of phosphorus = 31)
(d) Hydrochloric acid, HCl
(e) Nitric acid, HNO3

Ans: The molar mass of the following are as: [Unit is ‘g’]
(a) Ethyne, C2H2 = 2 x 12 + 2 x 1 = 24 + 2 = 26 g
(b) Sulphur molecule, S8 = 8 x 32 = 256 g
(c) Phosphorus molecule, P4=4 x 31 = i24g
(d) Hydrochloric acid, HCl = 1 x 1 + 1 x 35.5 = 1 + 35.5 = 36.5 g
(e) Nitric acid, HN03 = 1 x 1 + 1 x 14 + 3 x 16 = 1 + 14 + 48 = 63 g

Q7. What is the mass of
(a) 1 mole of nitrogen atoms?
(b) 4 moles of aluminium atoms (Atomic mass of aluminium = 27)?
(c) 10 moles of sodium sulphite (Na2S03)?

Ans:

(a) Mass of 1 mole of nitrogen atoms = 14 g

(b) 4 moles of aluminium atoms

Mass of 1 mole of aluminium atoms = 27 g
∴ Mass of 4 moles of aluminium atoms = 27 x 4 = 108 g

(c) 10 moles of sodium sulphite (Na2SO3)

Mass of 1 mole of Na2SO3 = 2 x 23 + 32 + 3 x 16 = 46 + 32 + 48 = 126 g
∴ Mass of 10 moles of Na2SO3 = 126 x 10 = 1260 g

Q8. Convert into mole.
(a) 12 g of oxygen gas
(b) 20 g of water
(c) 22 g of Carbon dioxide.

Ans: (a) Given mass of oxygen gas = 12 g
Molar mass of oxygen gas (O2) = 32 g
Mole of oxygen gas 12/32 = 0.375 mole
(b) Given mass of water = 20 g
Molar mass of water (H2O) = (2 x 1) + 16 = 18 g
Mole of water = 20/18 = 1.12 mole
(c) Given mass of Carbon dioxide = 22 g
Molar mass of carbon dioxide (CO2) = (1 x 12) + (2 x 16)
= 12 + 32 = 44 g
∴ Mole of carbon dioxide = 22/44 = 0.5 mole

Q9. What is the mass of:
(a) 0.2 mole of oxygen atoms?
(b) 0.5 mole of water molecules?

Ans: (a) Mole of Oxygen atoms = 0.2 mole
Molar mass of oxygen atoms = 16 g
Mass of oxygen atoms = 16 x 0.2 = 3.2 g
(b) Mole of water molecule = 0.5 mole
Molar mass of water molecules = 2 x 1 + 16= 18 g .
Mass of H2O = 18 x 0.5 = 9 g

Q10. Calculate the number of molecules of sulphur (S8) present in 16 g of solid sulphur.
Ans: Molar mass of S8 sulphur = 256 g = 6.022 x 1023 molecule
Given mass of sulphur = 16 g

NCERT Solutions For Class 9 Science Chapter 3 Atoms and Molecules Textbook Question 10

Q11. Calculate the number of aluminium ions present in 0.051 g of aluminium oxide. (Hint: The mass of an ion is the same as that of an atom of the same element. Atomic mass of Al = 27 u)

Ans: Molar mass of aluminium oxide Al203
= (2 x 27) + (3 x 16)
= 54 + 48 = 102 g.

NCERT Solutions For Class 9 Science Chapter 3 Atoms and Molecules Textbook Questions Question 11

 

 

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