Atoms and Molecules NCERT Questions with Answer

Atoms and Molecules
Class: 9 | Chapter: 3

NCERT Intext Questions & Answer

1. 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 ethonoate + Carbon dioxide + water
Answer: When in any reaction , Mass of reactants = Mass of products it means that reaction follows laws of conservation of mass. Here reactants are sodium carbonate and ethanoic acid and products are carbon dioxide, sodium ethanoate and water.
Mass of reactants = 5.3g (sodium carbonate) + 6g ( ethanoic acid) = 11.3 g And, Mass of products = 2.2 g (carbon dioxide) + 8.2 g ( sodium ethanoate) + 0.9 g (water ) = 11.3g
Both masses are equal. Hence, these observations are in agreement with the law of conservation of mass.

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 ?
Answer: The given ratio is 1 : 8 follows that
1 g Hydrogen requires 8 g of Oxygen
So, 3 g of Hydrogen will require , 8 × 3 = 24 g oxygen

3. Which postulate of Dalton’s atomic theory is the result of the law of conservation of mass ?
Answer: Atoms are indivisible, which can't be created or destroyed in a chemical reaction.

4. Which postulate of Dalton’s atomic theory can explain the law of definite proportions ?
Answer: The relative number and kinds of atoms are constant in a given compound.

5. Define the atomic mass unit.
Answer: One atomic mass unit is a mass unit equal to exactly one twelth the mass of one atom of carbon-12.

6. Why is it not possible to see an atom with naked eyes ?
Answer: Atoms are very small in size so, it is not possible to see an atom with naked eyes.

7. Write down the formulae of
(i) Sodium oxide
(ii) Aluminium chloride
(iii) Sodium sulphide
(iv) Magnesium hydroxide
Answer: (i) Sodium has valency 1 and oxide has valency 2 So, by cross multiplication of valencies, the formulae will be Na₂O.
(ii) Aluminium has valency 3 and chloride has valency 1 So, by cross multiplication of valencies, the formulae will be AlCl₃.
(iii) Sodium has valency 1 and sulphide has valency 2 So, by cross multiplication of valencies, the formulae will be Na₂S.
(iv) Magnesium has Valency 2 and hydroxide has valency 1 , So by cross multiplication of valencies, the formulae will be Mg(OH)₂.

8.Write down the names of compounds represented by the following formulae:
(i) Al₂(SO₄)₃
(ii) CaCl₂
(iii) K₂SO₄
(iv) KNO₃
(v) CaCO₃
Answer: (i) Aluminium sulphate
(ii) Calcium chloride
(iii) Potassium sulphate
(iv) Potassium nitrate
(v) Calcium carbonate

9. What is meant by the term chemical formula ?
Answer: It represents the composition of a molecule of the compound in terms of the symbols of the elements present in it.

10. How many atoms are present in a
(i) H₂S molecule and
(ii) P0₄^3- ion ?
Answer: (i) In H₂S, there are 2 hydrogen atoms and 1 sulphur atom. (ii) In P0₄^3- ion, there is 1 phosphorus atom and 4 oxygen atoms.

11. Calculate the molecular masses of H₂, O₂, Cl₂, C0₂, CH₄, C₂H₂,NH₃, CH₃OH.
Answer: Atomic mass of H = 1
So, Molecular mass of H₂ = 2 ×1= 2 u

Atomic mass of O = 16
So, Molecular mass of O₂ = 2 × 16 = 32 u

Atomic mass of Cl = 35.5
So, Molecular mass of Cl₂ = 2 × 35.5 = 71 u

Atomic mass of C = 12 , Atomic mass of O = 16
So, Molecular mass of CO₂ = 1 ×12 + 2 × 16 = 44 u

Atomic mass of C = 12 , Atomic mass of H = 1
So, Molecular mass of CH₄ = 1 ×12 + 4 × 1 = 16 u

As we know, Atomic masses of C, H are 12 and 1
So, Molecular mass of C₂H₄ = 2 ×12 + 1 × 4 = 28 u

Atomic mass of N = 14 , Atomic mass of H = 1
So, Molecular mass of NH₃ = 1 × 14 + 3 × 1 = 17 u

Atomic mass of C, H and O are 12,1 and 16
So, Molecular mass of CH₃OH = 1 × 12 + 4 × 1 + 1 × 16 = 32 u

12. 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.
Answer: Formula unit mass of ZnO is 65 + 16 = 81 u Formula unit mass of Na₂O is 2 × 23 + 1 ×16 = 62 u Formula unit mass of K₂CO₃ is 2 × 39 + 1 × 12 + 3 × 16 = 138 u

13. If one mole of carbon atoms weigh 12 grams, what is the mass (in grams) of 1 atom of carbon ?
Answer: 1 mole of carbon atoms = 6.022 × 10²³ atoms
According to que, 6.022 × 10²³ atoms weigh = 12 g
Then, 1 atom weigh = 12 / 6.022 × 10²³ = 1.99 × 10^-23 g

14. 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) ?
Answer: More is the no. of moles, more is the no. of particles (atoms).
(i) Number of moles in 100 gm of sodium = Given mass / Atomic mass = 100 / 23 = 4.3
(ii) Number of moles in 100 gm of iron = 100 / 56 = 1.8
Now, from the calculations it is clear that 100 gm of Na has more no. of moles and hence has more no. Of atoms.

NCERT Excercise Questions & Answer

1. 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.
Answer: Percentage composition = Given mass / Total mass of compound × 100 % Here we need to calculate percentage composition of Boron and Oxygen in compound.
So, Percentage Composition of Boron = (0.096 / 0.24) × 100 % => 40 % And Percentage composition of Oxygen = (0.144 / 0.24) × 100 % => 60 %

2. 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 ?
Answer: When 3 g of Carbon is burnt with 50 g of oxygen then also we get 11 g of CO₂ due to law of constant proportion.

3. What are poly atomic ions? Give examples.
Answer: A group of atoms carrying a charge is known as a polyatomic ion examples are Sulphate (SO₄)^2- and Carbonate (CO₃)^2- etc.

4. Write the chemical formulae of the following:
(a) Magnesium chloride
(b) Calcium oxide
(c) Copper nitrate
(d) Aluminium chloride
(e) Calcium carbonate.
Ans. (a) MgCl₂
(b) CaO
(c) Cu(NO₃) 2
(d) AlCl₃
(e) CaCO₃

5. Give the names of the elements present in the following compounds:
(a) Quick lime
(b) Hydrogen bromide
(c) Baking powder
(d) Potassium sulphate.
Answer: (a) Quick lime is CaO, elements are Ca and Oxygen
(b) Hydrogen bromide is HBr, elements are Hydrogen and Bromine
(c) Baking powder is NaHCO₃, elements are Sodium, Hydrogen, Carbon and Oxygen
(d) Potassium sulphate is K₂SO₄, elements are Potassium, Sulphur and Oxygen.

6. 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
Answer: (a) Atomic masses of C , H are 12,1
So, molar mass of C₂H₂ = 2 × 12 + 2 × 1 => 26 u

(b) Atomic mass of S is 32
So, molar mass of S₈ = 8 × 32 => 256 u

(c) Atomic mass of P is 31 So, molar mass of P₄ = 4 × 31 => 124 u

(d) Atomic masses of H and Cl are 1 , 35.5 respectively
So, molar mass of HCl = 1 + 35.5 => 36.5 u

(e) Atomic masses of H, N and O are 1,14 and 16 respectively
So, molar mass of HNO₃ = 1 + 14 + 3 × 16 => 63 u

7. 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 (Na₂S0₄) ?
Answer: (a) Gram atomic mass = Mass of 1 mole of atoms
Gram atomic mass of Nitrogen = Mass of 1 mole of Nitrogen atoms = 14 g

(b) Mass of 4 moles of Aluminium atoms = 4 × Gram atomic mass of Aluminium = 4 × 27 => 108 g

(c) Mass of 1 mole of Sodium sulphite = 2 × 23 + 32 + 3 × 16 => 126
Mass of 10 moles of Sodium sulphite = 10 × 126 => 1260 g

8. Convert into mole.
(a) 12 g of oxygen gas
(b) 20 g of water
(c) 22 g of Carbon dioxide.
Answer: As we know, No. Of Moles = Given mass / Atomic or Molecular mass
(a) No. of moles of Oxygen (O₂) = 12 / 2 × 16 => 0.375 mole
(b) No. of moles of water (H₂O) = 20 / 2 × 1+16 => 1.11 mole
(c) No. of moles of Carbon dioxide ( CO₂) = 22 / 12 + 2 × 16 => 0.5 mole

9. What is the mass of:
(a) 0.2 mole of oxygen atoms ?
(b) 0.5 mole of water molecules ?
Answer: No. Of moles = Given mass / Atomic or molecular mass
=> Given mass = No. Of moles × Atomic mass or mooecular mass
(a) No. of moles of oxygen = 0.2
Atomic mass of oxygen = 16
Therefore, mass of 0.2 mole of oxygen = 16 × 0.2 => 3.2 g

(b) No. of moles of water = 0.5
Molecular mass of water (H₂O) = 2 × 1 + 16 = 18
Therefore, mass of o. 5 mole of water molecules = 18 × 0.5 => 9 g

10. Calculate the number of molecules of sulphur (S₈) present in 16 g of solid sulphur.
Answer: No. of molecules = No. Of moles × 6.022 × 10²³
By applying above formulae, No. Of moles of S₈ in 16 g = 16 / Molecular mass of S₈( 8 × 32)
No. Of molecules = (16 / 256) × 6.022 × 10²³ => 3.76 × 10²³ molecules

11. 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)
Answer: In a molecule of aluminium oxide ( Al₂O₃) , there are 2 Al ions.
Therefore, 1 mole of Al₂O₃ contains 2 moles of Al ions.
No. Of moles of Al₂O₃ in 0.051 g = 0.051 / Molecular mass of Al₂O₃ ( 2 × 27 + 3 × 16 = 102)
No. Of molecules of Al₂O₃ in 0.051 g = 0.051 / 102 × 6.022 × 10²³
No. Of Al ions of Al₂O₃ in 0.051 g = 2 × 0.051 /102 × 6.022 × 10²³ => 6 × 10²⁰ ions