Best NCERT Solutions for Science Class 9 Chapter 3

Class 9 Chapter 3 – Atoms and Molecules

Best NCERT Solutions for Science Class 9 Chapter 3 – Atoms and Molecules. This chapter examines the atoms and molecules that are the building blocks of all stuff. Gaining an understanding of these ideas is crucial to understanding how substances interact, develop, and change.

Best NCERT Solutions for Science Class 9 Chapter 3 – Atoms and Molecules

Exercise 1

Question 1. In a reaction, 5.3g of sodium carbonate reacted with 6g of ethanoic acid. The products were 2.2g of carbon dioxide, 0.9g of water, and 8.2g of sodium ethanoate. Show that these observations are in agreement with the law of conservation of mass.

Sodium carbonate + ethanoic acid → sodium ethanoate + carbon dioxide + water

Answer:

Given, Mass of sodium carbonate = 5.3 g

Mass of ethanoic acid = 6 g

Mass of sodium ethanoate = 8.2 g

Mass of carbon dioxide = 2.2 g

Mass of water = 0.9 g

Now, total mass before the reaction

= (5.3 + 6) g = 11.3 g

And, total mass after the reaction =

(8.2 + 2.2 + 0.9) g = 11.3 g

-Pre-reaction total mass equals post-reaction total mass.

As a result, this is consistent with the mass conservation law.

Question 2. Hydrogen and oxygen combine in a 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:

In water, H:O (by mass) = $1 : 8$

The mass of oxygen gas required to react completely with 1 g of hydrogen gas = 8 g.

So,

the mass of oxygen gas required to react completely with 3 g of hydrogen gas =

(8 \times 3) g = 24 g

Question 3. Which postulate of Dalton’s atomic theory is the result of the law of conservation of mass?

Answer: “Atoms are indivisible particles, which can neither be created nor destroyed in a chemical reaction” is the thesis of Dalton’s atomic theory, which is predicated on the principle of conservation of mass.

Question 4. Which postulate of Dalton’s atomic theory can explain the law of definite proportions?

Answer: “The number and kind of atoms of each element in a given compound are fixed, and the elements are composed of atoms with fixed masses.” The law of definite proportion is explained in this way.

Best NCERT Solutions for Science Class 9 Chapter 3 – Atoms and Molecules

Exercise 2

Question 1. Define atomic mass unit.

Answer: An atomic mass unit is a unit of mass that is precisely one-twelfth the mass of a single carbon-12 atom. It can be expressed as “a.m.u.” or “u.”

Question 2. Why is it not possible to see an atom with the naked eye?

Answer: Due to the small size of an atom we cannot see them with the naked eye.

Best NCERT Solutions for Science Class 9 Chapter 3 – Atoms and Molecules

Exercise 3

Question 1. Write down the formulae of

(i) Sodium oxide

Answer: Sodium Oxide:

N a 2 O

(ii) Aluminium chloride

Answer: Aluminium chloride:

AlC l 3

(iii) sodium sulphide

Answer: Sodium sulphide:

N a 2 S

(iv) Magnesium hydroxide

Answer: Magnesium hydroxide: $Mg(OH)_{2}$

Question 2. Write down the names of compounds represented by the following formulae:

i) ${A l}_{2} ({S O}_{4})_{3}$

Answer: Aluminium sulphate

ii) $CaC l_{2}$

Answer: Calcium chloride

iii) $K_{2} S O_{4}$

Answer: Potassium sulphate

iv) $KN O_{3}$

Answer: Potassium nitrate

v) $CaC O_{3}$

Answer: Calcium carbonate

Question 3. What is meant by the term chemical formula?

Answer: The chemical formula is a symbolic representation of a compound’s makeup. The chemical formula provides an estimate of the total number of atoms.

For example , using the chemical formula CO2

We learn about carbon dioxide that a single molecule of the compound is made up of two oxygen atoms and one carbon atom bound together by a chemical bond.

Question 4. How many atoms are present in a:

i) $H_{2}$ S molecule

Answer: There are total 3 atoms present in $H_{2} S$ molecule, two hydrogen atoms and one Sulphur atom.

ii) ${PO}_{4}^{3 -}$ ion

Answer: There are total 5 atoms in

PO 3 - 4

ion, one phosphorus atom and 4 oxygen atoms.

Best NCERT Solutions for Science Class 9 Chapter 3 – Atoms and Molecules

Exercise 4

Question 1. Calculate the molecular masses of

H 2, O 2, C l 2, C O 2, C H 4, C 2 H 6, C 2 H 4, N H 3, C H 3 OH .

Answer: Molecular mass of $H_{2}$ = $2 \times$ Atomic mass of H

$= 2 \times 1 u = 2 u$

Molecular mass of $O_{2}$ = $2 \times$ Atomic mass of O

$= 2 \times 16 u = 3 2 u$

Molecular mass of $C l_{2}$ = $2 \times$ Atomic mass of Cl

$= 2 \times 35.5 u = 71 u$

Molecular mass of $C O_{2}$ = Atomic mass of C $+$ $2 \times$ Atomic mass of O

$= (12 + 2 \times 16) u = 44 u$

Molecular mass of $C H_{4}$ = Atomic mass of C $+$ $4 \times$ Atomic mass of H

$= (12 + 4 \times 1) u = 16 u$

Molecular mass of $C_{2} H_{6}$ = $2 \times$ Atomic mass of C $+$ $6 \times$ Atomic mass of H

$= (2 \times 12 + 6 \times 1) u = 30 u$

Molecular mass of $C_{2} H_{4}$ = $2 \times$ Atomic mass of C $+$ $4 \times$ Atomic mass of H

$= (2 \times 12 + 6 \times 1) u = 30 u$

Molecular mass of $N H_{3}$ = Atomic mass of N $+$ $3 \times$ Atomic mass of H

$= (14 + 3 \times 1) u = 17 u$

Molecular mass of $C H_{3} OH$ = Atomic mass of C $+$ $3 \times$ Atomic mass of H $+$ Atomic mass of O

$+$ Atomic mass of $= (12 + 4 \times 1 + 16) u = 32 u$

Question 2. Calculate the formula unit masses of

ZnO, N a 2 O, K 2 C O 3

given atomic masses of

Z = 65 u, Na = 23 u, K = 39 u, C = 12 u, and O = 16 u .

Ans: Formula unit mass of ZnO = Atomic mass of Zn + Atomic mass of O

= (65 + 16) u = 81 u

Formula unit mass of $N a_{2} O$ = $2 \times$ Atomic mass of Na + Atomic mass of O

= $(2 \times 23 + 16) u = 62 u$

Formula unit mass of $K_{2} C O_{3}$ = $2 \times$ Atomic mass of K + Atomic mass of C + $3 \times$ Atomic mass O=

$= (2 \times 39 + 12 + 3 \times 16) u$

$= 138 u$

Best NCERT Solutions for Science Class 9 Chapter 3 – Atoms and Molecules

Refer to page 42.

Question 1. If one mole of carbon atoms weighs 12 gram, what is the mass (in gram) of 1 atom of carbon?

Answer: Given mass of One mole of carbon atoms = 12 g

Therefore , Mass of

6.022 \times 1023

number of carbon atoms = 12 g

Mass of 1 atom of carbon will be:

$= \frac{12}{6.022 \times 10^{23}} g$

$= 1.9927 \times 10^{- 23} g$

Question 2. 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: Atomic mass of Na = 23 u (Given)

Then, gram atomic mass of Na = 23 g

Now, 23 g of Na contains =

6.022 \times 1023

number of Na atoms

Thus, 100 g of Na contains =

6.022 \times 10 23 23 \times 100

number of Na atoms

= 2.6182 \times 1024 number of Na atoms

Atomic mass of Fe = 56 u (Given)

Then, gram atomic mass of Fe = 56 g

Now, 56 g of Fe contains =

6.022 \times 1023

number of Fe atoms

Thus, 100 g of Fe contains =

6.022 \times 10 23 56 \times 100

number of Fe atoms

= 1.0753 \times 1024 number of Fe atoms

2.6182 \times 1024 > 1.0753 \times 1024

Therefore, 100 grams of sodium contain a greater number of atoms than 100 grams of iron.

Best NCERT Solutions for Science Class 9 Chapter 3 – Atoms and Molecules

NCERT QUESTIONS:

Question 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:

Given,

Mass of boron = 0.096 g

Mass of oxygen = 0.144 g

Mass of sample = 0.24 g

The percentage of boron by weight in the compound

$= \frac{0.096}{0.24} \times 100 % = 40 %$

And, percentage of oxygen by weight in the compound

$= \frac{0.144}{0.24} \times 100 % = 60 %$

Question 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 combinations will govern your answer?

Answer: Carbon + Oxygen ⎯⎯→ Carbon dioxide

3 g of carbon reacts with 8 g of oxygen to produce 11 g of carbon dioxide.

If 3 g of carbon is burnt in 50 g of oxygen, then 3 g of carbon will react with 8 g of oxygen to form11 g of carbon dioxide.

The remaining (50 –8) = 42 g of oxygen will be left unreacted.

The above answer is governed by the law of constant proportions.

Question 3. What are polyatomic ions? Give examples?

Answer: A polyatomic ion is a group of atoms carrying a charge either positive or negative.

For example,

$ammonium ion ({NH}_{4}^{+}), hydroxide ion (O H^{-}), carbonate ion, ({CO}_{3}^{2 -}), sulphate ion ({SO}_{4}^{2 -})$

Question 4. Write the chemical formulae of the following:

(a) Magnesium chloride

Answer:

$MgC l_{2}$

(b) Calcium oxide

Answer: CaO

(c) Copper nitrate

Answer:

$Cu {(N O_{3})}_{2}$

(d) Aluminium chloride

Answer:

$AlC l_{3}$

(e) Calcium carbonate

Answer:

$CaC O_{3}$

Question 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:

Compound	Chemical Formula	Elements Present
Quick Lime	$CaO$	Calcium, Oxygen
Hydrogen Bromide	$HBr$	Hydrogen, Bromine
Baking Powder	$NaHC O_{3}$	Sodium, Hydrogen, Carbon, Oxygen
Potassium Sulphate	$K_{2} S O_{4}$	Potassium, Sulphur, Oxygen

Question 6. Calculate the molar mass of the following substances:

a) Ethyne $C_{_{2}} H_{2}$

Answer: Molar mass of

$C_{2} H_{2} = 2 \times 12 + 2 \times 1 = 28 g / mol$

b) Sulphur molecule, $S_{8}$

Answer: Molar mass of

$S_{8} = 8 \times 32 = 256 g / mol$

c) Phosphorus molecule P4P4

(atomic mass of phosphorus = 31)

Answer: Molar mass of

$P_{4} = 4 \times 31 = 124 g / mol$

c) Hydrochloric acid, HCl

Answer: Molar mass of

$HCl = 1 + 35.5 = 36.5 g / mol$

d) Nitric acid,HNO3HNO3

Answer: Molar mass of

$HN O_{3} = 1 + 14 + 3 \times 16 = 63 g / mol$

Question 7. What is the mass of:

(a) 1 mole of nitrogen atoms?

Answer: The mass of 1 mole of N- atoms = 14 g

(b) 4 moles of aluminium atoms (Atomic mass of aluminium = 27)?

Answer:

$[Atomic mass of Al = 27 u]$

The mass of 4 moles of Al-atoms

$= (4 \times 27) g = 108 g$

(c) 10 moles of sodium sulphite

$(N a_{2} S O_{3})$ ?

Answer: Atomic mass of Na = 23 u, Atomic mass of S = 32 u, Atomic mass of O = 16 u

The mass of 10 moles of sodium sulphite

$(N a_{2} S O_{3})$

$= 10 \times [2 \times 23 + 32 + 3 \times 16] g$

$= 10 \times 126 g = 1260 g$

Best NCERT Solutions for Science Class 9 Chapter 3 – Atoms and Molecules

Introduction to the Structure of the Atom

1. What is an Atom?

An atom is the smallest unit of matter that retains the properties of an element. Everything around us is made up of atoms. They are the building blocks of all matter.

2. Basic Parts of an Atom

Atoms are made up of three main types of subatomic particles:

Protons: Positively charged particles found in the nucleus of the atom.
Neutrons: Neutral particles (no charge) also located in the nucleus.
Electrons: Negatively charged particles that orbit around the nucleus.

3. Structure of the Atom

Nucleus: The central part of the atom, made up of protons and neutrons. It is dense and contains most of the atom’s mass.
Electron Cloud: The area surrounding the nucleus where electrons are likely to be found. Electrons orbit the nucleus in various energy levels or shells.

4. Atomic Number and Mass Number

Atomic Number: The number of protons in the nucleus of an atom. It determines the element’s identity and its position in the periodic table.
Mass Number: The total number of protons and neutrons in the nucleus.

5. Isotopes

Atoms of the same element with different numbers of neutrons are called isotopes. They have the same atomic number but different mass numbers.

6. Electron Configuration

Electrons are arranged in shells around the nucleus. The arrangement of electrons in these shells is known as electron configuration. For example, the first shell can hold up to 2 electrons, the second up to 8, and so on.

7. Bohr Model

The Bohr model is a way of visualizing the atom with electrons orbiting the nucleus in fixed paths or shells. This model helps to understand the electron arrangement and energy levels.