Samacheer Kalvi 10th Science Solutions Chapter 7 Atoms and Molecules

Samacheer Kalvi 10th Science Solutions Chapter 7 Atoms and Molecules

Tamilnadu Samacheer Kalvi 10th Science Solutions Chapter 7 Atoms and MoleculesQuestions and Answers

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Samacheer Kalvi 10th Science Atoms and Molecules Textual Solved Problems

I. Calculation of molar mass:

Atoms And Molecules Class 10 Samacheer Question 1.
Calculate the gram molar mass of the following.
(i) H₂O
(ii) CO₂
(iii) Ca₃(PO₄)₂
Solution:
(i) H₂O
Atomic masses of H = 1, O = 16
Gram molar mass of H₂O = (1 × 2) + (16 × 1) = 2 + 16
Gram molar mass of H₂O = 18 g.

(ii) CO₂
Atomic masses of C = 12, O = 16
Gram molar mass of CO₂ = (12 × 1) + (16 × 2) = 12 + 32
Gram molar mass of CO₂ = 44 g.

(iii) Ca₃(PO₄)₂
Atomic masses of Ca = 40, P = 30, O = 16.
Gram molar mass of Ca₃(PO₄)₂ = (40 × 3) + [30 + (16 × 4)] × 2
= 120 + (94 × 2)
= 120 + 188
Gram molar mass of Ca₃(PO₄)₂ = 308 g.

Question 2.
Atoms of different elements having the same number of _____ are called isotones.
Answer:
Neutrons.

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

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

Question 5.
Relative atomic mass is otherwise known as _____.
Answer:
Standard atomic weight.

Question 6.
The average atomic mass of hydrogen is _____ amu.
Answer:
1.008 amu.

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

Question 8.
The number of atoms present in a molecule is called its _____.
Answer:
Atomicity.

Question 9.
One mole of any gas occupies _____ ml at S.T.P.
Answer:
22400.

Question 10.
Atomicity of phosphorous is _____.
Answer:
4.

III. Match the following.

Question 1.

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

Answer:
a – ii, b – iii, c – v, d – i, e – iv.

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

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

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

Question 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.

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

Question 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.

Question 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.

Question 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.

(vii) By substituting the relative molecular mass value in vapour density definition, we get
Vapour density (V.D.) = Relative molecular mass / 2
⇒ 2 × vapour density = Relative molecular mass of a gas.

VIII. HOT Questions

Question 1.

Calcium carbonate is decomposed on heating in the following reaction CaCO₃ → CaO + CO₂
(i) How many moles of Calcium carbonate are involved in this reaction?
Answer:
One mole

(ii) Calculate the gram molecular mass of calcium carbonate involved in this reaction.
Answer:
Gram molecular mass of CaCO₃
= 40 + 12 + 3(16)
= 100 g

(iii) How many moles of CO₂ are there in this equation?
Answer:
One mole.

IX. Solve the following problems.

Question 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
Answer:
Mass of 2 moles of H₂ molecule
= 2 × 2 = 4 g

(ii) 3 moles of chlorine molecule, Cl₂
Answer:
Gram molecular mass of 3 moles of Cl₂
= 3 × 71 = 213 g

(iii) 5 moles of sulphur molecule, S₂
Answer:
Gram molecular mass of 5 moles of S₂
= 5 × 8(32)
= 5 × 256 = 1280 g

(iv) 4 moles of phosphorous molecule, P₄
Answer:
Gram molecular mass of 4 moles of P₂
= 4 × 4(31)
= 4 × 124 = 496 g

⇒ 10x + 11 – 11x = 10.804 amu
⇒ 11 – x = 10.804 amu
⇒ -x = 10.804 – 11
⇒ -x = -0.196
⇒ x = 0.196
x = % abundance of B – 10 = 0.196 × 100 = 19.6 %
y = % abundance of B – 11 = 100 – 19.6 = 80.4 %
Percentage abundance of B – 10 = 19.6 %
Percentage abundance of B – 11 = 80.4 %.

Activities

Question 1.
Complete the following table by filling the appropriate values / terms

Solution:

Question 2.
Classify the following molecules based on their atomicity and fill in the table:
Fluorine (F₂), Carbon dioxide (CO₂), Phosphorous (P₄), Sulphur (S₈), Ammonia (NH₃), Hydrogen iodide (HI), Sulphuric Acid (H₂SO₄), Methane (CH₄), Glucose (C₆H₁₂O₆), Carbon monoxide (CO)

Solution:

Question 3.
Under same conditions of temperature and pressure if you collect 3 litres of O₂, 5 litres of Cl₂ and 6 litres of H₂,

1. Which has the highest number of molecules?
2. Which has the lowest number of molecules?

Question 8.
The atom with no neutrons in the nucleus is:
(a) He
(b) Deuterium
(c) Tritium
(d) Protium
Answer:
(d) Protium

Question 9.
The average atomic mass of carbon is _____.
(a) 12 amu
(b) 12.84 amu
(c) 24.011 amu
(d) 12.011 amu.
Answer:
(d) 12.011 amu.

Question 10.
Which one of the following is the most abundant element in both the Earth’s crust and in the human body?
(a) Carbon
(b) Silicon
(c) Oxygen
(d) Hydrogen.
Answer:
(c) Oxygen

Question 11.
Gram molecular mass of H₂SO₄ is:
(a) 49 g
(b) 54 g
(c) 98 g
(d) 100 g
Answer:
(c) 98 g

Question 12.
Boron – 10 and Boron – 11 are called _____.
(а) isotopes
(b) isobars
(c) isotones
(d) isomers.
Answer:
(c) isotopes

Question 13.
Which of the following are found in the elementary state in nature?
(a) Hydrogen chloride
(b) Carbon dioxide
(c) Noble gases
(d) Oxygen.
Answer:
(c) Noble gases

Question 14.
Ammonia gas is formed by the following reaction
N₂(g) + 3H₂(g) → 2NH₃(g)
The volume of H₂ required to form 6 dm³ of NH₃ is:
(a) 9 dm³
(b) 10 dm³
(c) 4 dm³
(d) 2 dm³
Answer:
(a) 9 dm³

Question 15.
Which one of the following is a hetero diatomic molecule?
(a) O₂
(6) N₂
(c) HI
(d) CH₄.
Answer:
(c) HI

Question 16.
Which one of the following is a hetero triatomic molecule?
(a) H₂O
(b) BCl₃
(c) CH₄
(d) PCl₅.
Answer:
(a) H₂O

Question 17.
1 gm atom of nitrogen represents:
(a) 6.023 × 10² N₂ molecules
(b) 22.4 litre of N₂ at STP
(c) 11.2 L of N₂ at STP
(d) 28 g of nitrogen
Answer:
(c) 11.2 L of N₂ at STP

Question 18.
Find out the hetero diatomic molecule?
(a) Hydrogen
(b) Hydrogen chloride
(c) Methane
(d) Ammonia.
Answer:
(b) Hydrogen chloride

Question 19.
Which one of the following is an example of a polyatomic molecule?
(a) Sulphur
(b) Gold
(c) Sodium
(d) Helium.
Answer:
(a) Sulphur

Question 20.
The gram molar mass of CO₂ is:
(a) 44 g
(b) 100 g
(c) 4.4 g
(d) 22 g
Answer:
(a) 44 g

Question 21.
Which one of the following is an example of a polyatomic molecule?
(a) Fluorine
(b) Glucose
(c) Oxygen
(d) Sodium.
Answer:
(b) Glucose (C₆H₁₂O₆)

Question 22.
The gram molecular mass of water is _____.
(a) 18 amu
(b) 18 g
(c) 18 u
(d) 18.
Answer:
(b) 18 g

Question 23.
The value of Avogadro’s number is _____.
(a) 6.023 × 10^-23
(b) 6.023 × 10²³
(c) 22.4
(d) 22400.
Answer:
(b) 6.023 × 10²³

Question 24.
The value of molar volume is _____.
(a) 22.4 ml
(b) 22.4 litres
(c) 22400 litres
(d) 2.24 litres.
Answer:
(b) 22.4 litres

Question 28.
2 × Vapour density is equal to _____.
(a) atomic mass
(b) valency
(c) relative molecular mass
(d) atomic number.
Answer:
(c) relative molecular mass

Question 29.
The value of gram molar mass of CO₂ is _____.
(a) 44 amu
(b) 44 g
(c) 44
(d) 44 kg.
Answer:
(b) 44 g
Hint: Molar mass = 12 + (16 × 2) = 44 g.

Question 43.
Number of atoms which a molecule to sulphur contains is _____.
(a) 3
(b) 8
(c) 4
(d) 2.
Answer:
(b) 8 (S₈)

Question 44.
An example of a triatomic molecule is _____.
(a) Ozone
(b) Nitrogen
(c) Hydrogen
(d) Ammonia.
Answer:
(a) Ozone

Question 45.
The atomic mass of sodium is 23. The number of moles in 46 g of sodium is _____.
(a) 0.5
(b) 2
(c) 1
(d) 0.25.
Answer:
(b) 2
Hint:

Hint:

Question 51.
The atomicity of K₂Cr₂O₇ is _____.
(a) 9
(b) 11
(c) 10
(d) 12.
Answer:
(b) 11

Question 52.
All noble gases are _____ molecules.
(a) diatomic
(b) triatomic
(c) mono atomic
(d) poly atomic.
Answer:
(c) mono atomic

Question 53.
The total number of atoms represented by the compound CuSO₄ . 5H₂O is ____.
(a) 27
(b) 21
(c) 5
(d) 8.
Answer:
(b) 21

Question 54.
Which one of the following represents the mass of 0.5 moles of water molecules?
(a) 18 g
(b) 1.8 g
(c) 9 g
(d) 4.5 g.
Answer:
(c) 9 g
 Mole = Mass / Molecular mass 
Mass = Mole × Molecular mass = 0.5 × 18 = 9 g.

Question 4.

1. _____ are the building blocks of matter.
2. ______ is a triatomic molecule.

Answer:

1. Atoms and molecules
2. Ozone

Question 5.
NH₃, H₂O are _____ molecules whereas N₂, O₂ are _____ molecules.
Answer:
Heteroatomic, Homoatomic

Question 6.
____ and ____ are polyatomic molecules.
Answer:
Phosphorous (P₄), Sulphur (S₈)

Question 7.

1. Atoms of the same element with same atomic number but a different mass number are called _____.
2. Atoms of different elements with the same number of neutrons are called _____.

Answer:

1. Isotopes
2. Isotones

Question 8.
Atomicity of Nitrogen is _____ whereas the atomicity of Helium is _____.
Answer:
2, 1.

Question 9.
Atoms of the same element with same atomic number but having different mass number are called _____.
Answer:
Isotopes.

Question 10.
Atoms of different elements with the same atomic mass but a different atomic number are called _____.
Answer:
Isobars.

Question 11.
Atoms of different elements having the same number of neutrons but a different atomic number and different mass number are called _____.
Answer:
Isotones.

Question 12.
_____ is the smallest particle that takes part in the chemical reaction.
Answer:
Atom.

Question 13.
Anything that has mass and occupies space is called _____.
Answer:
Matter.

Question 14.
Protons and neutrons have considerable mass, but _____ don’t have considerable mass.
Answer:
Atoms.

Question 15.
_____ is one-twelfth of the mass of C – 12 atom, an isotope of carbon which contains _____ protons and ____ neutrons.
Answer:
The atomic mass unit, 6, 6.

Question 16.
_____ are the building blocks of matter.
Answer:
Atoms.

Question 17.
The stable isotope of _____ is used as the standard for measuring the relative atomic mass of an element.
Answer:
Carbon C – 12.

Question 18.
Modem methods of determination of atomic mass by _____ use C – 12 as standard.
Answer:
Mass Spectrometry.

Question 19.
The relative atomic mass of sulphur is _____.
Answer:
32.

Question 20.
The average atomic mass of carbon is ______.
Answer:
12.011 amu.

Question 21.
The average atomic mass of an element becomes fractional due to the presence of ______.
Answer:
Isotopes.

Question 22.
_____ is the most abundant element in both the Earth’s crust and the human body.
Answer:
Oxygen.

Question 23.
Except for _____ atoms of most of the elements are found in the combined form with itself or atoms of other elements.
Answer:
Noble gases.

Question 24.
A molecule is a combination of two or more atoms held together by _____.
Answer:
Chemical bonds.

Question 25.
If the molecule is made of similar kind of atoms, it is called ______.
Answer:
Homo atomic molecule.

Question 26.
The molecule that consists of atoms of different elements is called _____.
Answer:
Hetero atomic molecule.

Question 27.
The number of _____ present in the molecule is called its atomicity.
Answer:
atoms.

Question 28.
The atomicity of ozone is _____.
Answer:
3.

Question 29.
The atomicity of hydrogen chloride is _____.
Answer:
2.

Question 30.
Water is a _____ molecule.
Answer:
Hetero triatomic.

Question 31.
One mole of an element contains ______ atoms and it is equal to its gram atomic mass.
Answer:
6.023 × 10²³

Question 32.
One mole of any gas occupies ______ or _____ at S.T.P.
Answer:
22.4 litre, 22400 ml.

Question 33.
The _____ is useful to determine the empirical formula and molecular formula.
Answer:
Percentage composition.

Question 34.
The percentage composition of elements is useful to determine _____ and _____.
Answer:
Empirical formula, molecular formula.

Question 35.
Avogadro’s law is in agreement with ______.
Answer:
Dalton’s atomic theory.

Question 36.
_____ determines the relation between molecular mass and vapour density.
Answer:
Avogadro’s law.

Question 37.
Relative molecular mass is equal to _____.
Answer:
2 × Vapour density.

Question 38.
Atomicity of sulphur is _____.
Answer:
8.

Question 39.
The metals Cu, Ag, Au are _____ elements.
Answer:
Monoatomic.

Question 40.
The atomicity of H₂SO₄ is ______.
Answer:
7.

Question 41.
Atomicity of an element is equal to _____.
Answer:
 Molecular mass / Atomic mass 

III. Match the following.

Question 1.

i. Monoatomic molecule	(a) Ozone
ii. Diatomic molecule	(b) Phosphorous
iii. Triatomic molecule	(c) Helium
iv. Polyatomic molecule	(d) Oxygen

Answer:
i – c, ii – d, iii – a, iv – b.

Question 2.

i. 22.4 litres	(a) Avogadro Number
ii. 6.023 × 1023	(b) Molar volume
iii. 2 × vapour density	(c) 1 mole
iv. Mass / Atomic mass	(d) Molecular mass

Answer:
i – b, ii – a, iii – d, iv – c.

Question 3.

i. 17Cl35,17Cl37	(a) Isotones
ii. 6Cl13,7N14	(b) Isobars
iii. 18Ar40,20Ca40	(c) E = mc2
iv. Einstein’s equation	(d) Isotopes

Answer:
i – d, ii – a, iii – b, iv – c.

Question 4.

i. H2O	(a) 180 g
ii. NH3	(b) 44 g
iii. CO2	(c) 17 g
iv. C6H12O6	(d) 18 g

Answer:
i – d, ii – c, iii – b, iv – a.

Question 5.

i. NH3, CH4	(a) Polyatomic molecule
ii. O2, N2	(b) Monoatomic molecule
iii. He, Ne	(c) Heteroatomic molecule
iv. Sulphur	(d) Diatomic molecule

Answer:
i – c, ii – d, iii – b, iv – a.

Question 6.

i. F2	(a) Polyatomic molecule
ii. O3	(b) Monoatomic molecule
iii. P4	(c) Diatomic molecule
iv. He	(d) Triatomic molecule

Answer:
i – c, ii – d, iii – a, iv – b.

Question 7.

i. H2	(a) Hetero diatomic molecule
ii. HCl	(b) Monoatomic molecule
iii. H2O	(c) Homo diatomic molecule
iv. Ne	(d) Hetero triatomic molecule

Answer:
i – c, ii – a, iii – d, iv – b.

Question 8.

i. Isotopes	(a) S8, P4
ii. Isobars	(b) 6C13,7N14
iii. Isotones	(c) 1H1,1H2,1H3
iv. Polyatomic molecule	(d) 18Ar40,20Ca40

Answer:
i – c, ii – d, iii – b, iv – a.

Question 9.

i. H2O	(a) 16
ii. CO2	(b) 18
iii. C6H12O6	(c) 44
iv. CH4	(d) 180

Answer:
i – b, ii – c, iii – d, iv – a

Question 10.

i. 22 g of CO2	(a) 2 moles
ii. 18 g of H2O	(b) 4 moles
iii. 360 g of Glucose	(c) 0.5 mole
iv. 64 g of CH4	(d) 1 mole

Answer:
i – c, ii – d, iii – a, iv – b.

IV. State whether true or false. If false, give the correct statement.

Question 1.
Isotopes are the atoms of the same element may not be similar in all respects.
Answer:
True.

Question 2.
Isobars are the atoms of the different elements with the same atomic number and different mass numbers.
Answer:
False.
Correct statement: Isobars are the atoms of the different elements with the same mass number but a different atomic number.

Question 3.
Isotones are the atoms of different elements with the same number of neutrons.
Answer:
True.

Question 4.
The number of molecules present in one mole of an element is called atomicity of an element.
Answer:
False.
Correct statement: The number of atoms present in one molecule of an element is called the atomicity of an element.

Question 5.
Avogadro’s hypothesis is used in the deduction of atomicity of elementary gases.
Answer:
True.

Question 6.
The volume of a gas at a given temperature and pressure is proportional to the number of particles.
Answer:
True.

Question 7.
The value of Gram molar volume at STP is 11.2 litres.
Answer:
False.
Correct statement: The value of Gram molar volume at STP is 22.4 litres.

Question 8.
The atomicity of nitrogen, oxygen and hydrogen is two.
Answer:
True.

Question 9.
Atoms and molecules are the building blocks of matter.
Answer:
True.

Question 10.
The atoms of certain elements such as hydrogen, oxygen and nitrogen have an independent existence.
Answer:
False.
Correct statement: The atoms of certain elements such as hydrogen, oxygen and nitrogen do not have an independent existence.

Question 11.
A molecule is the simplest structural unit of an element or compound which contains one or more atoms.
Answer:
True.

Question 12.
Phosphorous and sulphur are monoatomic molecules.
Answer:
False.
Correct statement: Phosphorous and sulphur are polyatomic molecules.

Question 13.
H₂O, NH₃, CH₄ are examples of homoatomic molecules.
Answer:
False.
Correct statement: H₂O, NH₃, CH₄ are examples of heteroatomic molecules.

Question 14.
An atom of one element can be transmuted into an atom of other element is known as artificial transmutation.
Answer:
True.

Question 15.
The molecule is the smallest particle that takes part in a chemical reaction.
Answer:
False.
Correct statement: Atom is the smallest particle that takes part in a chemical reaction.

Question 16.
The sum of the number of protons and neutrons of an atom is called Atomic number.
Answer:
False.
Correct statement: The sum of the number of protons and neutrons of an atom is called mass number.

Question 17.
The stable isotope of carbon (C – 12) with atomic mass 12 is used as the standard for measuring the relative atomic mass of an element.
Answer:
True.

Question 18.
The gram atomic mass of oxygen is 16 g.
Answer:
True.

Question 19.
Silicon is the most abundant element in the Earth’s crust.
Answer:
False.
Correct statement: Oxygen is the most abundant element in the Earth’s crust.

Question 20.
Except for noble gases, atoms of most of the elements are found in the combined form.
Answer:
True.

Question 21.
The number of atoms present in the molecule is called the Avogadro number.
Answer:
False.
Correct statement: The number of atoms present in the molecule is called its Atomicity.

Question 22.
O₂, N₂, H₂, Cl₂, Br₂, F₂, I₂ are hetero diatomic molecules.
Answer:
False.
Correct statement: O₂, N₂, H₂, Cl₂, Br₂, F₂, I₂ are homo diatomic molecules.

Question 23.
Water is an example of Hetero triatomic molecule.
Answer:
True.

Question 24.
One molecule of an element contains 6.023 × 10²³ atoms and it is equal to its gram atomic mass.
Answer:
True.

Question 25.
An equal volume of all gases under similar conditions of temperature and pressure contain a different number of molecules.
Answer:
False.
Correct statement: Equal volume of all gases under similar conditions of temperature and pressure contain the same number of molecules.

Question 26.
The mathematical representation of Avogadro’slawisV/n=Constant(or)Vccn(or) V = Constant × n.
Answer:
True.

Question 27.
The molecular formula of gases can be derived using Avogadro’s law.
Answer:
True.

Question 28.
The number of moles of a sample that contains 12.046 x 10²³ atoms of iron is 2.
Answer:
True.

Question 29.
The volume occupied by 14 g of Nitrogen gas is 22.4 litres.
Answer:
False.
Correct statement: The volume occupied by 14 g of Nitrogen gas is 11.2 litres.

Question 30.
Avogadro’s law determines the relation between molecular mass and absolute density.
Answer:
False.
Correct statement: Avogadro’s law determines the relation between molecular mass and vapour density.

V. Assertion and Reason

Question 1.
Assertion (A): C₁₂H₂₂O₁₁ is not a simple ratio.
Reason (R): The ratio of atoms in a molecule may be fixed and integral but may not be simple.
(a) Both (A) and (R) are correct
(b) Both (A) and (R) are wrong
(c) (A) is correct but (R) is wrong
(d) (A) is wrong but (R) is correct.
Answer:
(a) Both (A) and (R) are correct

Question 7.
Assertion (A): 18 g water contains Avogadro number (6.023 × 10²³) of particles.
Reason (R): 18 g of water is the molecular mass (or) 1 mole of water. One mole is defined as the amount of the substance which contains 6.023 × 10²³ number of particles.
(a) (A) is correct and (R) explains (A)
(b) (A) is correct but (R) is wrong
(c) (A) is wrong but (R) is correct
(d) Both (A) and (R) are wrong
Answer:
(a) (A) is correct and (R) explains (A)

Question 8.
Assertion (A): Atoms of the same element may not be similar in all respects.
Reason (R): Atoms of the same element have the same atomic number but a different number of neutrons.
(a) Both (A) and (R) are correct
(b) (A) is correct but (R) is wrong
(c) Both (A) and (R) are wrong
(d) (A) is wrong but (R) is correct.
Answer:
(b) (A) is correct but (R)is wrong

Question 9.
Assertion (A): The atomicity of ozone is three.
Reason (R): 1 molecule of ozone contains 3 atoms of oxygen.
(a) Both (A) and (R) are correct
(b) Both (A) and(R) are wrong
(c) (A) is correct but (R) is wrong
(d) (A) is wrong but (R) is correct.
Answer:
(a) Both (A) and (R) are correct

Question 8.
What is homo atomic molecule? Give two examples.
Answer:
If the molecule is made of similar kind of atoms, then it is called homoatomic molecule. e.g. H₂, Cl₂

Question 9.
What is a heteroatomic molecule? Give two examples.
Answer:
The molecule that consists of atoms of different elements is called a heteroatomic molecule. e.g. HCl, H₂O

Question 10.
Consider the following and classify them on the basis of their atomicity.
H₂, CCl₄, O₃, BF₃, HCl, HNO₃, C₁₂H₂₂O₁₁, NO, Cl₂, He, Au, P₄

• Monoatomic molecule – He, Au
• Homo diatomic molecule – H₂, Cl₂
• Homo triatomic molecule – O₃
• Homo polyatomic molecule – P₄
• Hetero diatomic molecule – HCl, NO
• Hetero polyatomic molecule – CCl₄, BF₃, HNO₃, C₁₂H₂₂O₁₁.

Question 11.
Define Relative molecular mass.
Answer:
The Relative molecular mass of a molecule is the ratio between the mass of one molecule of the substance to 1 / 12th mass of an atom of Carbon – 12 isotope.

Question 12.
Define Mole.
Answer:
The mole is the amount of the substance that contains as many elementary entities as there are atoms in exactly 12 g of the carbon – 12 isotope.

Question 13.
Define the Avogadro number.
Answer:
The actual number of atoms in 12 g of carbon – 12 is called the Avogadro number.
It is equal to 6.023 × 10²³ (N_A).

Question 14.
What is meant by percentage composition? What is its use?
Answer:
The percentage composition of a compound represents the mass of each element present in 100 g of the compound. It is useful to determine the empirical formula and molecular formula.

Question 15.
State Avogadro hypothesis (or) Avogadro’s Law.
Answer:
The Avogadro’s law states that “equal volume of all gases under similar conditions of temperature and pressure contain the equal number of molecules”.
[V ∝ n].

Question 16.
What are the applications of Avogadro’s Law?
Answer:

• It explains Gay – Lussac’s law.
• It helps in the determination of atomicity of gases.
• The molecular formula of gases can be derived using Avogadro’s law.
• It determines the relation between molecular mass and vapour density.
• It helps to determine the gram molar volume of all gases, (i.e, 22.4 litres at S.T.P).

Question 17.
How is Average atomic mass calculated?
Answer:
The average atomic mass of an element is calculated by adding the masses of its isotopes, each multiplied by their natural abundance on the Earth.

Question 18.
Define Vapour density.
Answer:
The vapour density is defined as the ratio between the masses of equal volumes of a gas (or vapour) and hydrogen under the same condition.

Question 19.
Write the relationship between

1. Atomicity and Molecular mass
2. Molecular mass and Vapour density.

Answer:

1. 1 mole of C₂H₆ contains 2 moles of carbon atoms
   3 moles of C₂H₆ will C – atoms = 6 moles
2. 1 mole of C₂H₆ contains 6 moles of hydrogen atoms
   3 moles of C₂H₆ will contain H-atoms = 18 moles
3. 1 mole of C₂H₆ contains Avogadro’s number. i.e., 6.023 × 10²³ molecules.
   3 moles of C₂H₆ will contain ethane molecules = 3 × 6.023 × 10²³ = 18.06 × 10²³ molecules.

Question 5.
If ten volumes of dihydrogen gas react with five volumes of dioxygen gas, how many volumes of water vapour could be produced?
Answer:
H₂ and O₂ react according to the equation
H_{2 (g)} + O_{2 (g)} → 2H₂O _(g)
Thus, 2 volumes of H₂ react with 1 volume of O₂ to produce 2 volumes of water vapour.
Hence, 10 volumes of H₂ will react completely with 5 volumes of O₂ to produce 10 volumes of water vapour.

VIII. Long Answer Questions.

Question 1.
What are the differences between atoms and molecules?
Answer:

Atom	Molecule
An atom is the smallest particle of an element	A molecule is the smallest particle of an element or compound.
Atom does not exist in the free state except in a noble gas	The molecule exists in the free state
Except some of the noble gas, other atoms are highly reactive	Molecules are less reactive
Atom does not have a chemical bond	Atoms in a molecule are held by chemical bonds
Example: Na	Example: N2

Question 2.
Write the applications of Avogadro’s Law.
Answer:
(i) It explains Gay-Lussac’s law.
(ii) It helps in the determination of atomicity of gases.
(iii) Molecular formula of the gases can be derived.
(iv) It determines the relation between molecular mass and vapour density.
(v) It helps to determine gram molar volume of all gases

Question 3.
State and explain the applications of Avogadro’s law.
(OR)
Give any two applications of Avogadro’s law.
(OR)
Write any three applications of Avogadro’s law.
Answer:
Avogadro’s law: Equal volumes of all gases under the same conditions of temperature and pressure contain an equal number of molecules.
Applications of Avogadro’s law:

• It is used to determine the atomicity of gases.
• It is helpful in determining the molecular formula of gaseous compounds.
• It establishes the relationship between the vapour density and molecular mass of a gas.
• It gives the value of the molar volume of gases at STP. Molar volume of a gas at STP = 22.4 litres.
• It explains Gaylussac’s law effectively.

Question 4.
Explain the classification of molecules based on atomicity.
Answer:
In accordance with the number of atoms present in the molecules, they are classified as monoatomic, diatomic, triatomic and polyatomic molecules showing that they contain one, two, three or more than 3 atoms respectively.

Atomicity	Number of atoms per molecule	Example
Monoatomic molecule	1	Helium (He), Neon (Ne) metals (Fe, Cu)
Diatomic molecule	2	Hydrogen (H2), Chlorine (Cl2)
Triatomic molecule	3	Ozone (O3)
Polyatomic molecule	>3	Phosphorous (P4), Sulphur (S8)

Question 5.
A compound made up of two elements A and B has A = 70%, B = 30%. Their relative number of moles in the compound are 1.25 and 1.88. Calculate.
(a) Atomic masses of the elements A and B.
(b) The molecular formula of the compound, if its molecular mass is found to be 160.

Answer:

Question 4.
Find the gram molecular mass of the following from the data given:
(i) H₂O
(ii) CO₂
(iii) NaOH
(iv) NO₂
(v) H₂SO₄

Element	Symbol	Atomic No.	Atomic Mass
Hydrogen	H	1	1
Carbon	C	6	12
Oxygen	O	8	16
Nitrogen	N	7	14
Sodium	Na	11	23
Sulphur	S	16	32

Solution:
(i) H₂O
Atomic mass of 2(H) = 2 × 1 = 2
Atomic mass of 1(O) = 1 × 16 = 16
Molecular mass of H₂O = 2 + 16 = 18

(ii) CO₂
Atomic mass of 1(C) = 1 × 12 = 12
Atomic mass of 2(O) = 2 × 16 = 32
Molecular mass of CO₂ = 12 + 32 = 44 g

(iii) NaOH
Atomic mass of 1(Na) = 1 × 23 = 23
Atomic mass of 1(O) = 1 × 16 = 16
Atomic mass of 1(H) 1 × 1 = 1
Molecular mass of NaOH = 23 + 16 + 1 = 40 g

(iv) NO₂
Atomic mass of 1(N) = 1 × 14 = 14
Atomic mass of 2(O) = 2 × 16 = 32
Molecular mass of NO₂ = 14 + 32 = 46 g.

(v) H₂SO₂₄
Atomic mass of 2(H) = 2 × 1= 2
Atomic mass of 1(S) = 1 × 32 = 32
Atomic mass of 4(O) = 4 × 16 = 64
Molecular mass of H₂SO₄ = 64 + 32 + 2 = 98 g.

Question 5.
Complete the table given below.

Element	Atomic Mass	Molecular Mass	Atomicity
Chlorine	35.5	71	–
Ozone	–	45	3
Sulphur	32	–	8

Solution:

Element	Atomic Mass	Molecular Mass	Atomicity
Chlorine	35.5	71	2
Ozone	16	48	3
Sulphur	32	256	8

Question 6.
Fill in the blanks using the given data:
The formula of Calcium oxide is CaO. The atomic mass of Ca is 40, Oxygen is 16 and Carbon is 12.

• 1 mole of Ca (….. g) and 1 mole of the Oxygen atom (…… g) combine to form mole of CaO (….. g).
• 1 mole of Ca (…… g) and 1 mole of C (…… g) and 3 moles of the Oxygen atom (…… g) combine to form 1 mole of CaCO₃ (…… g).

Solution:

• 1 mole of Ca (40 g) and 1 mole of the Oxygen atom (16 g) combine to form 1 mole of CaO (56 g).
• 1 mole of Ca (40 g) and 1 mole of C (12 g) and 3 moles of the Oxygen atom (48 g) combine to form 1 mole of CaCO₃ (100 g).
• 

Solution:

Gas	Atomic mass	Molecular mass	Atomicity
Ozone	16	48	3
Nitrogen	14	28	2

Question 9.
Analyse the table and fill in the blanks.

Substance	Mass	No.of moles
(a) Al	81 g	–
(b) Fe	–	0.5

Solution:

Substance	Mass	No.of moles
(a) Al	81 g	3
(b) Fe	27.95 g	0.5

Question 10.
When ammonia reacts with hydrogen chloride gas, it produces white fumes of ammonium chloride. The volume occupied by NH₃ in glass bulb A is three times more than the volume occupied by HCl in glass bulb B at STP.

(i) How many moles of ammonia are present in glass bulb A?
(ii) How many grams of NH₄Cl will be formed when the stopper is opened? (Atomic mass of N = 14, H = 1, Cl = 35.5)
(iii) Which gas will remain after completion of the reaction?
(iv) Write the chemical reaction involved in this process.
Solution:

Question 14.
How many grams are there in the following?
(i) 1 mole of chlorine molecule, Cl₂
(ii) 2 moles of sulphur molecules, S₈
(iii) 4 moles of ozone molecules, O₃
(iv) 2 moles of nitrogen molecules, N₂
Solution:
(i) 1 mole of chlorine molecule Cl₂
Atomic mass of chlorine = 35.5 g
Mass of 1 mole of chlorine = Atomic mass × Atomicity = 35.5 × 2 = 71 g.

(ii) 2 moles of sulphur molecules S₈
Atomic mass of S₈ = 8 × 32 = 256 g
Mass of 2 moles of S₈ = Atomic mass × Number of moles = 256 × 2 = 512 g.

(iii) 4 moles of ozone molecule O₃
Atomic mass of O₃ = 3 × 16 = 48 g
Mass of 4 moles of ozone = 48 × 4 = 192 g.

(iv) 2 moles of Nitrogen molecule N₂
Atomic mass of N₂ = 2 × 14 = 28 g
Mass of 2 moles of Nitrogen = 28 × 2 = 56 g.

Question 15.
Find how many moles of atoms are there in:
(i) 2 g of nitrogen
(ii) 23 g of sodium
(iii) 40 g of calcium
(iv) 1.4 g of lithium
(v) 32 g of sulphur.

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