3. Atoms and Molecules Class 9 NCERT Exemplar Solutions Chapter 3 (CBSE Science)

CBSE Class 9 Science Chapter 3 : Atoms and Molecules – NCERT Exemplar Questions with Answers

Chapter 3: Atoms and Molecules

Multiple Choice Questions

1. Which of the following correctly represents 360 g of water?

(i) 2 moles of (ii) 20 moles of water (iii) molecules of water (iv) molecules of water

(a) (i) (b) (i) and (iv) (c) (ii) and (iii) (d) (ii) and (iv)

Answer: (d) (ii) and (iv)

[ Molar mass of water ()= 2 × 1 + 1×16 = 2+16 = 18 g

We have,

Number of molecules molecules ]

2. Which of the following statements is not true about an atom?

(a) Atoms are not able to exist independently

(b) Atoms are the basic units from which molecules and ions are formed

(c) Atoms are always neutral in nature

(d) Atoms aggregate in large numbers to form the matter that we can see, feel or touch

Answer: (a) Atoms are not able to exist independently

[ This statement is not true because some atoms such as noble gases (helium, neon, argon, etc.) can exist independently. ]

3. The chemical symbol for nitrogen gas is

(a) Ni (b) (c) (d) N

Answer: (b)

[ The chemical symbol for nitrogen gas is N₂ because it naturally exists as a diatomic molecule.]

4. The chemical symbol for sodium is

(a) So (b) Sd (c) NA (d) Na

Answer: (d) Na .

[ The chemical symbol for sodium is Na. ]

5. Which of the following would weigh the highest?

(a) 0.2 mole of sucrose ( )

(b) 2 moles of

(c) 2 moles of

(d) 10 moles of

Answer: (c) 2 moles of (200 g)

[ (a) 0.2 mole of sucrose ( ):

Molar mass = 12×12 + 22×1 + 11×16 = 144 + 22 + 176 = 342 g

Mass = 0.2 × 342 = 68.4 g

(b) 2 moles of :

Molar mass = 12 + 16×2 = 44 g

Mass = 2 × 44 = 88 g

Molar mass = 40 + 12 + 16×3 = 40 + 12 + 48 = 100 g

Mass = 2 × 100 = 200 g

(d) 10 moles of :

Molar mass = 18 g

Mass = 10 × 18 = 180 g ]

6. Which of the following has maximum number of atoms?

(a) 18g of H₂O (b) 18g of O₂ (c) 18g of CO₂ (d) 18g of CH₄

Answer: (d) 18 g of CH₄

[ We have, Moles = mass ÷ molar mass

Molar mass of H₂O = 18 g

Moles = 18 ÷ 18 = 1 mole

Atoms per molecule = 3 (= 2 + 1)

Total atoms = 1 × 6.022×10²³ × 3 = 1.8066 × 10²⁴ atoms

Molar mass of O₂ = 32 g

Moles = 18 ÷ 32 = 0.5625 mole

Atoms per molecule = 2

Total atoms = 0.5625 × 6.022×10²³ × 2 = 6.77 × 10²³ atoms

Molar mass of CO₂ = 12 + 16×2 = 44 g

Moles = 18 ÷ 44 ≈ 0.409 mole

Atoms per molecule = 3 (=1 + 2)

Total atoms = 0.409 × 6.022×10²³ × 3 = 7.39 × 10²³ atoms

Molar mass of CH₄ = 12 + 4 = 16 g

Moles = 18 ÷ 16 = 1.125 moles

Atoms per molecule = 5 (=1+ 4)

Total atoms = 1.125 × 6.022×10²³ × 5 ≈ 3.39 × 10²⁴ atoms

7. Which of the following contains maximum number of molecules ?

(a) 1g CO₂ (b) 1g (c) 1g (d) 1g CH₄

Answer: (c) 1 g H₂

[ We have, Moles = mass ÷ molar mass

Molar mass of CO₂ = 12 + 16 × 2 = 44 g

Moles = 1 ÷ 44 = 0.0227 mol

Molar mass of N₂ = 14 × 2 = 28 g

Moles = 1 ÷ 28 = 0.0357 mol

Molar mass of H₂ = 2 g

Moles = 1 ÷ 2 = 0.5 mol

Molar mass of CH₄ = 12 + 4 = 16 g

Moles = 1 ÷ 16 = 0.0625 mol

8. Mass of one atom of oxygen is

(a) (b) (c) (d) 8u

Answer: (a)

Atomic mass = 16 g

Avogadro’s number =

Mass of one O atom

9. 3.42 g of sucrose are dissolved in 18g of water in a beaker. The number of oxygen atoms in the solution are

(a)

(b)

(c)

(d)

Answer: (a) .

[Mass of sucrose (C₁₂H₂₂O₁₁) = 3.42 g

Molar mass of sucrose (C₁₂H₂₂O₁₁) = 12 × 12 + 1 × 22 +11 × 16 =144 + 22 + 176 = 342 g

Mole

So, oxygen atoms from sucrose

Mass of water (H₂O) = 18 g

Molar mass of H₂O = 18 g

Mole

So, oxygen atoms from water

Total moles of oxygen atoms = 1+0.11 = 1.11

Number of oxygen atoms ]

10. A change in the physical state can be brought about

(a) only when energy is given to the system

(b) only when energy is taken out from the system

(c) when energy is either given to, or taken out from the system

(d) without any energy change

Answer: (c) When energy is either given to, or taken out from the system.

[ A change in physical state (like melting, boiling, freezing) happens by adding or removing energy:

Heating → solid → liquid → gas

Cooling → gas → liquid → solid ]

Short Answer Questions

11. Which of the following represents a correct chemical formula? Name it.

(a) CaCl (b) (c) (d) NaS

Answer: (a) CaCl
Valency of Calcium = 2, Valency Chlorine = −1.
Correct formula = CaCl2 ; CaCl is incorrect.

(b) BiPO₄
Valency of Bismuth = 3 , Valency of phosphate PO₄³⁻ = 3
Correct formula: (b) BiPO₄
Name: Bismuth(III) phosphate (or bismuth phosphate).

Correct formula = Na₂SO₄, So NaSO₄ is incorrect.

(d) NaS
Valency of Sodium = 1 and Valency of Sulphur = 2

Correct formula = Na₂S, So NaS is incorrect.

12. Write the molecular formulae for the following compounds

(a) Copper (II) bromide

(b) Aluminium (III) nitrate

(c) Calcium (II) phosphate

(d) Iron (III) sulphide

(e) Mercury (II) chloride

(f) Magnesium (II) acetate

Answer: (a) Copper (II) bromide - CuBr₂
(b) Aluminium (III) nitrate - Al(NO₃)₃
(c) Calcium (II) phosphate - Ca₃(PO₄)₂
(d) Iron (III) sulphide - Fe₂S₃
(e) Mercury (II) chloride - HgCl₂
(f) (f) Magnesium (II) acetate - Mg(CH₃COO)₂

13. Write the molecular formulae of all the compounds that can be formed by the combination of following ions

Answer: Positive ions (cations): Cu²⁺ Na⁺ , Fe³⁺

Negative ions (anions): Cl⁻ , SO₄²⁻ , PO₄³⁻

Copper(II) compounds :

Cu²⁺ + Cl⁻ → CuCl₂
Cu²⁺ + SO₄²⁻ → CuSO₄
Cu²⁺ + PO₄³⁻ → Cu₃(PO₄)₂

Sodium compounds :

Na⁺ + Cl⁻ → NaCl
Na⁺ + SO₄²⁻ → Na₂SO₄
Na⁺ + PO₄³⁻ → Na₃PO₄

Iron(III) compounds :

Fe³⁺ + Cl⁻ → FeCl₃
Fe³⁺ + SO₄²⁻ → Fe₂(SO₄)₃
Fe³⁺ + PO₄³⁻ → FePO₄

14. Write the cations and anions present (if any) in the following compounds

(a) CH₃COONa (b) NaCl (c) H₂ (d) NH₄NO₃

Answer: (a) CH₃COONa

Cation: Na⁺

Anion: CH₃COO⁻ (acetate ion)

(b) NaCl

Cation: Na⁺

Anion: Cl⁻

Cation: None

Anion: None

(d) NH₄NO₃

Cation: NH₄⁺

Anion: NO₃⁻

15. Give the formulae of the compounds formed from the following sets of elements

(a) Calcium and fluorine (b) Hydrogen and sulphur (c) Nitrogen and hydrogen (d) Carbon and chlorine (e) Sodium and oxygen (f) Carbon and oxygen

Answer: (a) Calcium and Fluorine :

Ca F

Valency : 2 1

Formula = CaF₂ (Calcium fluoride)

(b) Hydrogen and Sulphur :

H S

Valency 1 2

Formula = H₂S (Hydrogen sulphite)

N H

Valency 3 1

Formula = NH₃ (Ammonia)

(d) Carbon and Chlorine :

C Cl

Valency 4 1

Formula = CCl₄ (Carbon tetrachloride)

(e) Sodium and Oxygen :

Na O

Valency 1 2

Formula = Na₂O (Sodium oxide)

(f) Carbon and Oxygen :

C O

Valency 4 2

Cross the valencies: C₂O₄ ( dividing by 2)

Formula = CO₂ (Carbon dioxide)

16. Which of the following symbols of elements are incorrect? Give their correct symbols

(a) Cobalt CO (b) Carbon c (c) Aluminium AL (d) Helium He (e) Sodium So

Answer: (a) Cobalt — CO Incorrect
Correct symbol of Cobalt is Co
(Capital C, small o)

(b) Carbon — c Incorrect ( First letter must be capital.)
Correct symbol is C .

(d) Helium — He Correct ( First letter capital, second letter small.)

(e) Sodium — So Incorrect ( Symbol of Sodium comes from its Latin name Natrium.)
Correct symbol is Na

17. Give the chemical formulae for the following compounds and compute the ratio by mass of the combining elements in each one of them. (You may use appendix-III).

(a) Ammonia (b) Carbon monoxide (c) Hydrogen chloride (d) Aluminium fluoride (e) Magnesium sulphide

Answer: (a) Chemical formula of ammonia is NH₃ .

Atomic masses: N = 14 u, H = 1 u

Mass ratio: N : H = 14 : (3×1) = 14 : 3

(b) Chemical formula of Carbon monoxide is CO

Atomic masses: C = 12 u, O = 16 u

Mass ratio: C : O = 12 : 16 = 3 : 4 (dividing by 4)

Atomic masses: H = 1 u, Cl = 35.5 u

Mass ratio: H : Cl = 1 : 35.5

(d) Chemical formula of Aluminium fluoride is AlF₃

Atomic masses: Al = 27 u, F = 19 u

Mass ratio: Al : F = 27 : (3×19) = 27 : 57 = 9 : 19 (divide by 3)

(e) Chemical formula of Magnesium sulphide is MgS

Atomic masses: Mg = 24 u, S = 32 u

Mass ratio: Mg : S = 24 : 32 = 3 : 4 (dividing by 8)

18. State the number of atoms present in each of the following chemical species

(a) (b) (c) P₂O₅ (d) CO

Answer: (a) CO₃²⁻

Total atoms = 1 (C) + 3 (O) = 4 atoms

(b) PO₄³⁻

Total atoms = 1 (P) + 4 (O) = 5 atoms

Total atoms = 2 (P) + 5 (O) = 7 atoms

(d) CO

Total atoms = 1 (C) + 1 (O) = 2 atoms

19. What is the fraction of the mass of water due to neutrons?

Answer: Formula of water is H₂O

Mass of hydrogen = 1 (proton) + 0 (neutron)

Mass of oxygen= 8 (protons) + 8 (neutrons)

Total nucleons (mass number) of water = 2×1 (H)+16 (O) = 2 + 16 = 18 u
Total neutrons = 8

20. Does the solubility of a substance change with temperature? Explain with the help of an example.

Answer: Yes, solubility of a substance changes with temperature.

For most solid substances, solubility increases when temperature increases. For example, more sugar dissolves in hot water than in cold water. So, temperature directly affects how much solute can dissolve in a solvent.

21. Classify each of the following on the basis of their atomicity.

(a) F₂ (b) NO₂ (c) N₂O (d) C₂H₆ (e) P₄ (f) H₂O₂ (g) P₄O₁₀ (H) O₃ (i) HCl (j) CH₄ (k) He (l) Ag

Answer: Atomicity is the number of atoms present in the molecule of a substance.

(a) Atomicity of F₂ is 2 atoms → Diatomic

(b) Atomicity of NO₂ is 3 (= 1 N + 2 O) → Triatomic

(d) Atomicity of C₂H₆ is 8 (= 2 C + 6 H) → Polyatomic

(e) Atomicity of P₄ is 4 → Polyatomic

(f) Atomicity of H₂O₂ is 4 (= 2 H + 2 O) → Tetraatomic

(g) Atomicity of P₄O₁₀ is 14 (= 4 P + 10 O) → Polyatomic

(h) Atomicity of O₃ is 3 → Triatomic

(i) Atomicity of HCl is 2 (1 H + 1 O) → Diatomic

(j) Atomicity of CH₄ is 5 (= 1 C + 4 H) = 5 → Polyatomic .

(k) Atomicity of He is 1 → Monatomic

(l) Atomicity of Ag is 1 → Monatomic

22. You are provided with a fine white coloured powder which is either sugar or salt. How would you identify it without tasting?

Answer: Take a little of the white powder in a test tube and heat it gently. If it is sugar, it will melt, turn brown and give a burnt smell because it decomposes on heating. If it is salt, it will not change colour or smell and will remain white even after heating.

23. Calculate the number of moles of magnesium present in a magnesium ribbon weighing 12 g. Molar atomic mass of magnesium is 24 g/mol .

Answer: Given, Mass of magnesium ribbon = 12 g

Molar atomic mass of magnesium = 24 g/mol

Long Answer Questions

24. Verify by calculating that

(a) 5 moles of CO₂ and 5 moles of H₂O do not have the same mass.

(b) 240 g of calcium and 240 g magnesium elements have a mole ratio of 3:5.

Answer: (a) 5 moles of CO₂ and 5 moles of H₂O do not have the same mass

Molar mass of CO₂ = 12 + 2 × 16 = 12 + 32 = 44 g/mol

Molar mass of H₂O = 2 × 1 + 16 = 2 + 16 = 18 g/mol

Mass of 5 moles CO₂ = 5 × 44 = 220 g

Mass of 5 moles H₂O = 5 × 18 = 90 g

So, they do not have the same mass.

(b) 240 g calcium and 240 g magnesium have a mole ratio 3 : 5

Molar mass of calcium (Ca) = 40 g/mol

Molar mass of magnesium (Mg) = 24 g/mol

25. Find the ratio by mass of the combining elements in the following compounds. (You may use Appendix-III)

(a) CaCO₃ (b) MgCl₂ (c) H₂SO₄ (d) C₂H₅OH (e) NH₃ (f) Ca(OH)₂

Answer: Atomic masses : H = 1 , C = 12, O = 16, N = 14, S = 32, Mg = 24, Cl = 35.5, Ca = 40

(a) CaCO₃ : Mass of Ca = 40 ; Mass of C = 12 ; Mass of O = 16 × 3 = 48

Ca : C : O = 40 : 12 : 48 = 10 : 3 : 12

(b) MgCl₂ : Mass of Mg = 24 , Mass of Cl = 35.5 × 2 = 71

Mg : Cl = 24 : 71

H : S : O = 2 : 32 : 64 = 1 : 16 : 32

(d) C₂H₅OH : Mass of C = 12 × 2 = 24 ; Mass of H = 1 × 6 = 6
Mass of O = 16

C : H : O = 24 : 6 : 16 = 12 : 3 : 8

(e) NH₃ : Mass of N = 14 ; Mass of H = 1 × 3 = 3

N : H = 14 : 3

(f) Ca(OH)₂ : Mass of Ca = 40 ; Mass of O = 16 × 2 = 32 ; Mass of H = 1 × 2 = 2

Ca : O : H = 40 : 32 : 2 = 20 : 16 : 1

26. Calcium chloride when dissolved in water dissociates into its ions according to the following equation.

CaCl₂ (aq) → Ca²⁺ (aq) + 2Cl⁻ (aq)

Calculate the number of ions obtained from CaCl₂ when 222 g of it is dissolved in water.

Solution: The dissociation equation is: CaCl₂ (aq) → Ca²⁺ (aq) + 2Cl⁻ (aq)

Molar mass of CaCl₂ = 40 + 2 × 35.5 = 111 g/mol

Mass given = 222 g

∴ 2 moles of CaCl₂ = 2 × 3 = 6 moles of ions

And 6 moles of ions = 6 × 6.022 × 10²³ = 3.6132 × 10²⁴ ions

27. The difference in the mass of 100 moles each of sodium atoms and sodium ions is 5.48002 g. Compute the mass of an electron.

Answer: The difference in mass arises because 100 moles of Na⁺ ions have lost 100 moles of electrons compared to 100 moles of neutral Na atoms.

Mass of 100 moles of electrons = 5.48002 g

Number of electrons in 100 moles

So, mass of 1 moles of electrons

Then, mass of one electron = 9.10 × 10⁻²⁸ g .

28. Cinnabar (HgS) is a prominent ore of mercury. How many grams of mercury are present in 225 g of pure HgS ? Molar mass of Hg and S are 200.6 g/mol and 32 g/mol respectively.

Answer: Given, Mass of HgS = 225 g
Molar mass of Hg = 200.6 g/mol
Molar mass of S = 32 g/mol

Molar mass of HgS = 200.6 + 32 = 232.6 g/mol

Mass of

29. The mass of one steel screw is 4.11g. Find the mass of one mole of these steel screws. Compare this value with the mass of the Earth (5.98 × 1024 kg). Which one of the two is heavier and by how many times?

Solution: Given, Mass of one steel screw = 4.11 g

Mass of 1 mole

Mass of Earth

Therefore, Earth is heavier than this mass of screws by a factor of about 2416 .

30. A sample of vitamic C is known to contain oxygen atoms. How many moles of oxygen atoms are present in the sample?

Solution: Given, number of oxygen atoms = atoms

31. Raunak took 5 moles of carbon atoms in a container and Krish also took 5 moles of sodium atoms in another container of same weight. (a) Whose container is heavier? (b) Whose container has more number of atoms?

Solution: Given, Raunak: 5 moles of carbon atoms

Krish: 5 moles of sodium atoms

Molar mass of carbon =12 g/mol ; Molar mass of sodium =23 g/mol

(a) Mass in Raunak’s container:

Mass of C = 5 × 12 = 60 g

Mass in Krish’s container:

Mass of Na = 5 × 23 = 115 g

Therefore, 115 g > 60 g

So Krish’s container is heavier.

(b) Number of atoms = moles × NA

Both have the same number of moles = 5 mol.

Number of atoms =

Thus, they have the same number of atoms.

32. Fill in the missing data in the Table 3.1

Species			Na atom
Property
No. of moles	2	--------	--------	0.5
No. of particle	---------		--------	--------
Mass	36g	--------	115g	--------

Answer: The missing data is :

Species Property	H₂O	CO₂	Na atom	MgCl₂
No. of moles	2	0.5	5.0	0.5
No. of particles	1.2044×10²⁴	3.011×10²³	3.011×10²⁴	3.011×10²³
Mass	36g	22g	115g	47.6g

For H₂O : Molar mass of H₂O = 18 g/mol.
.

No. of particles = 2 mol × 6.022×10²³ particles/mol = 1.2044×10²⁴.

For CO₂ :

Molar mass CO₂ = 0.5 × 44 = 22 g.

For Na atoms : Moles = 115 / 23 = 5.0 mol.

No. of atoms = 5 × 6.022×10²³ = 3.011×10²⁴ atoms.

For MgCl₂ : Moles = 0.5 mol MgCl₂ , Molar mass MgCl₂ = 95.211 g/mol .
Mass = 0.5 × 95.2 = 47.6 g.

33. The visible universe is estimated to contain stars. How many moles of stars are present in the visible universe?

Solution: Given, number of stars in visible universe =

One mole of anything contains .

That means the visible universe contains about 0.0166 moles of stars.

34. What is the SI prefix for each of the following multiples and submultiples of a unit?

(a) (b) (c) (d) (e) (f)

Answer: The SI prefixes are:

Power of 10	Prefix	Symbol
(a)	kilo	k
(b)	deci	d
(c)	centi	c
(d)	micro	µ
(e)	nano	n
(f)	pico	p

35. Express each of the following in kilograms

(a) mg (b) 58.34 g (c) 0.584g (d)

Answer: (a)

(b)

(c)

(d)

36. What are ionic and molecular compounds? Give examples.

Answer: Ionic compounds are those compounds which are formed by transfer of electrons from one atom to another, resulting in the formation of oppositely charged ions.

Example: Sodium chloride (NaCl) : , Magnesium oxide (MgO) : .
Molecular compounds are those compounds which are formed by sharing of electrons between atoms.

Example: H₂O (Water), CO₂ (Carbon dioxide) .

37. Compute the difference in masses of one mole each of aluminium atoms and one mole of its ions. (Mass of an electron is ). Which one is heavier?

Solution: Given, mass of one electron

Mass of 1 mole of electrons

Mass of 3 moles of electrons

1 mole of aluminium atoms is heavier than 1 mole of Al³⁺ ions, because atoms contain 3 extra electrons.

38. A sample of ethane (C₂H₆) gas has the same mass as 1.5 ×1020 molecules of methane (CH₄). How many C₂H₆ molecules does the sample of gas contain?

Solution: Given, number of methane molecules

Molar mass of CH₄ = 12 + (1 × 4) = 16 u

Molar mass of C₂H₆ = (12 × 2) + (1 × 6) = 24 + 6= 30 u

Let, N be the number of molecules of C₂H₆ .

A/Q,

39. Fill in the blanks

(a) In a chemical reaction, the sum of the masses of the reactants and products remains unchanged. This is called ————.

(b) A group of atoms carrying a fixed charge on them is called ————.

(c) The formula unit mass of is ————.

(d) Formula of sodium carbonate is ———— and that of ammonium sulphate is ————.

Answer: (a) Law of Conservation of Mass

(b) polyatomic ion

[ Atomic masses: Ca = 40 u , P = 31 u , O = 16 u

Formula unit mass = 3(40) + 2[ 31 + 4 (16)] = 120 + 2(31+64) =120 + 2 × 95 = 120 + 190 = 310 u

Therefore, the formula unit mass of is 310 u. ]

(d) Na₂CO₃ and (NH₄)₂SO₄

40. Write the formulae for the following and calculate the molecular mass for each one of them.

(a) Caustic potash (b) Baking powder (c) Lime stone (d) Caustic soda (e) Ethanol (f) Common salt

Answer: (a) Caustic potash

Chemical name: Potassium hydroxide (KOH)
Molecular mass of KOH = 39 + 16 + 1 = 56 u

(b) Baking powder

Chemical name : Sodium hydrogen carbonate (NaHCO₃ )

Formula = NaHCO₃ .
Molecular mass of NaHCO₃ = 23 + 1 + 12 + 48 = 84 u

Chemical name: Calcium carbonate (CaCO₃)
Molecular mass = 40 + 12 + 48 = 100 u

(d) Caustic soda

Chemical name: Sodium hydroxide (NaOH)
Molecular mass = 23 + 16 + 1 = 40 u

(e) Ethanol

Chemical name : Ethanol ()
Molecular mass of = 24 + 6 + 16 = 46 u

(f) Common salt

Chemical name: Sodium chloride (NaCl)
Molecular mass = 23 + 35.5 = 58.5 u

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3. Atoms and Molecules Class 9 NCERT Exemplar Solutions Chapter 3 (CBSE Science)

CBSE Class 9 Science Chapter 3 : Atoms and Molecules – NCERT Exemplar Questions with Answers

Chapter 3: Atoms and Molecules

Multiple Choice Questions

Short Answer Questions

Long Answer Questions

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