TN 10th Science Guide

7. Atoms and Molecules

I. Choose the best answer.

1. Which of the following has the smallest mass?

(a) 6.023 × 10²³ atoms of He
(b) 1 atom of He
(c) 2 g of He
(d) 1 mole atoms of He.

2. Which of the following is a triatomic molecule?

(a) Glucose                  (b) Helium
(c) Carbon dioxide    (d) Hydrogen

3. The volume occupied by 4.4 g of CO₂ at S.T.P _____.

(a) 22.4 litre    (b) 2.24 litre
(c) 0.24 litre    (d) 0.1 litre.

4. Mass of 1 mole of Nitrogen atom is:

(a) 28 amu       (b) 14 amu
(c) 28 g            (d) 14 g

5. Which of the following represents 1 amu?

(a) Mass of a C – 12 atom
(b) Mass of a hydrogen atom
(c) 1/12th of the mass of a C – 12 atom
(d) Mass of O – 16 atom

6. Which of the following statement is incorrect?

(a) One gram of C – 12 contains Avogadro’s number of atoms.
(b) One mole of oxygen gas contains Avogadro’s number of molecules.
(c) One mole of hydrogen gas contains Avogadro’s number of atoms.
(d) One mole of electrons stands for 6.023 × 10²³ electrons.

7. The volume occupied by 1 mole of a diatomic gas at S.T.P is _____.

(a) 11.2 litre                (b) 5.6 litre
(c) 22.4 litre                (d) 44.8 litre.

8. In the nucleus of ₂₀Ca⁴⁰, there are_______.

(a) 20 protons and 40 neutrons
(b) 20 protons and 20 neutrons
(c) 20 protons and 40 electrons
(d) 40 protons and 20 electrons.

9. The gram molecular mass of oxygen molecule is:

(a) 16 g            (b) 18 g
(c) 32 g            (d) 17 g

10. 1 mole of any substance contains ______ molecules.

(a) 6.023 × 1023         (b) 6.023 × 10^-23
(c) 3.0115 × 10²³        (d) 12.046 × 10²³.

II. Fill in the blanks

1. Atoms of different elements having Same mass number, but different atomic numbers are called isobars.

2. Atoms of different elements having the same number of Neutrons are called isotones.

3. Atoms of one element can be transmuted into atoms of other elements by Artificial transmutation.

4. The sum of the numbers of protons and neutrons of an atom is called its Mass number.

5. Relative atomic mass is otherwise known as Standard atomic weight.

6. The average atomic mass of hydrogen is 1.008 amu..

7. If a molecule is made of similar kind of atoms, then it is called Homo atomic molecule.

8. The number of atoms present in a molecule is called its Atomicity.

9. One mole of any gas occupies 22400 ml at S.T.P.

10. Atomicity of phosphorous is 4.

III. Match the following.

a. 8 g of O2	0.25 moles
b. 4 g of H2	2 moles
c. 52 g of He	13 moles
d. 112 g of N2	4 moles
e. 35.5 g of Cl2	0.5 moles

IV. True or False: (If false give the correct statement)

1. Two elements sometimes can form more than one compound.
       Answer:    True.

2. Noble gases are Diatomic
       Answer:   False.
Correct Statement: Noble gases are monoatomic

3. The gram atomic mass of an element has no unit?
       Answer:    False.
Correct Statement: The gram atomic mass of an element is expressed in the unit grams.

4. 1 mole of Gold and Silver contain the same number of atoms?
      Answer:     True

5. The molar mass of CO₂ is 42 g?
      Answer:     False.
Correct Statement: The molar mass of CO₂ is (12 + 32) = 44 g.

V. Assertion and Reason:

Answer the following questions using the data given below:
(i) A and R are correct, R explains the A.
(ii) A is correct, R is wrong.
(iii) A is wrong, R is correct.
(iv) A and R are correct, R doesn’t explain A.

1. Assertion: Atomic mass of aluminium is 27
Reason: An atom of aluminium is 27 times heavier than 1 / 12th of the mass of the C – 12 atoms.
Answer:  (i) A and R are correct, R explains the A.

2. Assertion: The Relative Molecular Mass of Chlorine is 35.5 a.m.u.
Reason: The natural abundance of Chlorine isotopes are not equal.
Answer:  (i) A and R are correct, R explains the A.

VI. Short Answer Questions

1. Define Relative atomic mass.

Relative atomic mass of an element is the ratio between the average mass of its isotopes to 1 / 12th part of the mass of a carbon – 12 atom. It is denoted as A_n

2. Write the different types of isotopes of oxygen and its percentage abundance.

Isotopes of oxygen:

Isotope Mass (amu) % abundance	Isotope Mass (amu) % abundance	Isotope Mass (amu) % abundance
8O16	8O16	8O16
8O17	8O17	8O17
8O18	8O18	8O18
15.9949	15.9949	15.9949

3. Define Atomicity.

The number of atoms present in the molecule is called atomicity.

4. Give any two examples for heteroatomic molecules.

Heterodiatomic molecules.
e.g.,

• HCl
• NaCl.

5. What is Molar volume of a gas?

The volume occupied by one mole of any gas at STP is called molar volume. Its value is equal to 22.4 litre or 22400 ml or 22400 cm³ or 2.24 × 10^-2 m³.

6. Find the percentage of nitrogen in ammonia.

VII. Long Answer Questions.

1. Calculate the number of water molecule present in one drop of water which weighs 0.18 g.

2. N₂ + 3H₂ → 2NH₃ (The atomic mass of nitrogen is 14, and that of hydrogen is 1)

• 1 mole of nitrogen (____ g) + ____.
• 3 moles of hydrogen (____ g) → ____.
• 2 moles of ammonia (____ g).

Answer:
N₂ + 3H₂ → 2NH₃

• 1 mole of N₂ = 28 g
• 3 moles of H₂ = 6 g
• 2 moles of NH₃ = 34 g
  ⇒ 1 mole of nitrogen (28 g) + 3 moles of hydrogen (6 g) → 2 moles of Ammonia (34 g).

3. Calculate the number of moles in:
(i) 27 g of Al
(ii) 1.51 × 10²³ molecules of NH₄Cl
Answer:

4. Give the salient features of ‘Modern atomic theory’.
Answer:
(i) An atom is no longer indivisible.
(ii) Atoms of the same element may have different atomic mass.
Eg: isotopes ₁₇Cl³⁵, ₁₇Cl³⁷.
(ii) Atoms of different elements may have same atomic masses.
Eg: Isobars ₁₈Ar⁴⁰, ₂₀Ca⁴⁰.
(iv) Atoms of one element can be transmuted into atoms of other elements. An atom is no longer indestructible.
(v) Atoms may not always combine in a simple whole number ratio.
Eg: Glucose C₆H₁₂O₆ C : H : O = 6 : 12 : 6 or 1 : 2 : 1.
(vi) Atom is the smallest particle that takes part in a chemical reaction.
(vii) The mass of an atom can be converted into energy (E = mc²).

5. Derive the relationship between Relative molecular mass and Vapour density.
Answer:

Relative Molecular Mass : The ratio of Mass of one molecule of gas or vapour to the mass of one atom of hydrogen.

VIII. HOT Questions

1. Calcium carbonate is decomposed on heating in the following reaction CaCO₃ → CaO + CO₂

Answer:

(i) How many moles of Calcium carbonate are involved in this reaction?

One mole

(ii) Calculate the gram molecular mass of calcium carbonate involved in this reaction.

Gram molecular mass of CaCO₃
= 40 + 12 + 3(16)
= 100 g

(iii) How many moles of CO₂ are there in this equation?

One mole.

IX. Solve the following problems.

1. How many grams are there in the following?

Answer:

Formula = No. of moles (n) × (Gram molecular mass)

(i) 2 moles of hydrogen molecule, H₂

Mass of 2 moles of H₂ molecule
= 2 × 2 = 4 g

(ii) 3 moles of chlorine molecule, Cl₂

Gram molecular mass of 3 moles of Cl₂
= 3 × 71 = 213 g

(iii) 5 moles of sulphur molecule, S₂

Gram molecular mass of 5 moles of S₂
= 5 × 8(32)
= 5 × 256 = 1280 g

(iv) 4 moles of phosphorous molecule, P₄

Gram molecular mass of 4 moles of P₂
= 4 × 4(31)
= 4 × 124 = 496 g

2. Calculate the % of each element in calcium carbonate. (Atomic mass: C – 12, O – 16, Ca – 40)
Solution:

3. Calculate the % of oxygen in Al₂(SO₄)₃. (Atomic mass: Al – 12, O – 16, S – 32)
Solution:

4. Calculate the % relative abundance of B – 10 and B – 11, if its average atomic mass is 10.804 amu.
Solution: