NCERT Solutions for Class 9 Science Chapter 3 (In-text Exercise Questions)
Given below the class 9 science chapter 3 atoms and molecules solutions of in-text questions. These solutions help students understanding the fundamental concepts they also make it easy for students to quickly revise important topics before exams.
Exercise-3.2 (Page: 30)
Question 1. Define the atomic mass unit.
Answer:
An atomic mass unit is a unit of mass used to express the weight of subatomic particles, where one unit is equal to exactly one-twelfth the mass of a carbon-12 atom.
Question 2. Why is it not possible to see an atom with naked eyes?
Answer:
We can't see the atom with the naked eye because they are minuscule in nature. They are measured in nanometres. Also, except for noble gases, all atoms do not exist independently. They exist in the form of any compound.
Exercise 3.3 and 3.4 (Page: 34)
Question 1. Write down the formulae of
(i) sodium oxide
(ii) aluminium chloride
(iii) sodium sulphide
(iv) magnesium hydroxide
Answer:
The formulas of the given compounds are :
(i) sodium oxide :
$\Rightarrow Na_2O$
(ii) aluminium chloride:
$\Rightarrow AlCl_3$
(iii) sodium sulphide:
$\Rightarrow Na_2S$
(iv) magnesium hydroxide:
$\Rightarrow Mg(OH)_2$
Question 2. Write down the names of compounds represented by the following formulae:
(i) $Al_{2}(SO_{4})_{3}$
(ii) $CaCl_{2}$
(iii) $K_{2}SO_{4}$
(iv) $KNO_{3}$
(v) $CaCO_{3}$
Answer:
The Names of the following compounds are :
(i) $Al_{2}(SO_{4})_{3}$
=Aluminium Sulphate
(ii) $CaCl_{2}$
=Calcium Chloride
(iii) $K_{2}SO_{4}$
= Potassium Sulphate
(iv) $KNO_{3}$
= Potassium Nitrate
(v) $CaCO_{3}$
= Calcium Carbonate.
Question 3. What is meant by the term chemical formula?
Answer:
The chemical formula of a compound is a symbolic representation of its composition. For example, the chemical formula for common salt is NaCl, as it is made up of Sodium (Na) and Chlorine (Cl).
Q 4.(i) How many atoms are present in a
Answer:
$H_{2}S$ molecule has 2 atoms of Hydrogen and 1 atom of Sulphur.
and hence,
A total of 3 atoms are present in $H_{2}S$ molecule.
Question 4.(ii) How many atoms are present in a
Answer:
$PO_{4}^{3-}$ ion has one atom of Phosphorus and 4 atoms of Oxygen.
And hence,
A total of 5 atoms are present in $PO_{4}^{3-}$ ion.
Exercise-3.5.1-3.5.2 (Page: 35)
Question 1. Calculate the molecular masses of $H_{2}, O_{2},Cl_{2},CO_{2},CH_{4},C_{2}H_{6}, C_{2}H_{4}, NH_{3},CH_{3}OH$
Answer:
The molecular mass of $H_2$ :
= 2 $\times$Atomic mass of Hydrogen
= 2 $\times$1u
=2u.
The molecular mass of $O_2$ :
= 2 $\times$ Atomic mass of Oxygen
= 2 $\times$16u
= 32u.
The molecular mass of $Cl_2$ :
= 2 $\times$ Atomic mass of Chlorine
= 2 $\times$ 35.5uu
= 71u.
The molecular mass of $CO_2$ :
= Atomic mass of Carbon +2 $\times$ Atomic mass of Oxygen
= 12u + 2 $\times$16u
= 44u.
The molecular mass of $CH_4$ :
= Atomic mass of Carbon +4 $\times$Atomic mass of Hydrogen
= 12u + 4 $\times$1u
= 16u.
The molecular mass of $C_2H_6$ :
= 2 $\times$ Atomic mass of Carbon + 6 $\times$ Atomic mass of Hydrogen
= 2$\times$12u+ 6 $\times$1u
=24u+ 6u
= 30u.
The molecular mass of $C_2H_4$ :
= 2 $\times$ Atomic mass of Carbon + 64$\times$Atomic mass of Hydrogen
= 2$\times$12u+ 4$\times$1u
=24u+ 4u
= 28u.
The molecular mass of $NH_3$ :
= Atomic mass of Nitrogen + 3 $\times$Atomic mass of Hydrogen
= 14u+ 3 $\times$ 1u
= 17u.
The molecular mass of $CH_3OH$ :
= Atomic mass of Carbon +4$\times$Atomic mass of Hydrogen + Atomic mass of Oxygen
= 12u+ 4 $\times$ 1u + 16u
= 32u.
Question 2. Calculate the formula unit masses of ZnO, Na2O, K2CO3 , given atomic masses of Zn = 65 u, Na = 23 u, K = 39 u, C = 12 u, and O = 16 u
Answer:
Given,
the atomic mass of Zn = 65 u,
the atomic mass of Na = 23 u,
the atomic mass of K = 39 u,
the atomic mass of C = 12 u,
and the atomic mass of O = 16 u
Now,
Formula unit mass of ZnO = Atomic mass of Zinc + Atomic mass of O
= 65 u + 16 u
= 81 u.
Formula unit mass of Na2O = 2 $\times$ Atomic mass of Na+ Atomic mass of O
= 2 $\times$23 u + 16 u
= 62 u.
Formula unit mass of K2CO3 = 2$\times$ Atomic mass of K+ Atomic mass of C + 3 $\times$ Atomic mass of O
= 2 $\times$ 39 u + 12 u + 3 $\times$ 16 u
= 138 u.
NCERT Solutions Class 9 Science Chapter 3( Exercise Questions with Answers)
These solutions provide clear answers to all the exercise questions of atoms and molecules ncert solutions. They help students to understand how to solve numerical problems and explain concepts in simple language.
Question 1. A 0.24 g sample of a 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:
The total mass of the compound = 0.24 g
Mass of boron in the compound = 0.096 g
Mass of oxygen in the compound = 0.144 g
Now, as we know,
The percentage of an element in the compound :
$=\frac{total\:mass\:of\:element}{total\:mass\:of\:compound}\times 100$
So,
The percentage of Boron in the compound by weight :
$=\frac{0.096}{0.24}\times 100$
$=40\%$
The percentage of Oxygen in the compound by weight :
$=\frac{0.144}{0.24}\times 100$
$=60 \%$
Q 3. What are polyatomic ions? Give examples.
Answer:
Polyatomic ions are ions that contain more than one atom. These atoms can be of the same type or of a different type.
Some examples of polyatomic ions are NH4+ , OH- , SO42- , and SO32- .
Question 4. Write the chemical formulae of the following.
(a) Magnesium chloride
(b) Calcium oxide
(c) Copper nitrate
(d) Aluminium chloride
(e) Calcium carbonate
Answer:
The chemical formula of the given compounds is:
(a) Magnesium chloride
$=MgCl_2$
(b) Calcium oxide
$=CaO$
(c) Copper nitrate
$=Cu(NO_3)_2$
(d) Aluminium chloride
$=AlCl_3$
(e) Calcium carbonate
$=CaCO_3$
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:
The names of the elements present in the given compounds are :
(a) Quick lime :
$CaO$ = Calcium and Oxygen.
(b) Hydrogen bromide:
$HBr$ = Hydrogen and Bromine.
(c) Baking powder:
$NaHCO_3$ = Sodium, Hydrogen, Carbon, and Oxygen.
(d) Potassium sulphate:
$K_2SO_4$ = Potassium, Sulphur, and oxygen.
Question 6.(c) Calculate the molar mass of the following substances
Answer:
The molecular formula of Baking soda is NaHCO3
So the molecular mass will be some of the individual atoms involved in the formula.
Molecular Mass of baking soda = Molecular mass (Na + H + C + 3xO)
= 23 + 1 + 12 + 3x16
= 84 g/mol
Question 6. (e) Calculate the molar mass of the following substances.
Answer:
The molecular mass of Nitric acid, HNO3 = 1 $\times$ Atomic Mass of N + 1 $\times$ Atomic Mass of H + 3 $\times$ Atomic Mass of O
= 1$\times$ 14u + 1 $\times$1u + 3 $\times$ 16u
= 14u + 1u + 48u
= 63u
Practice Questions for Class 9 Science Chapter 3
The class 9 science chapter 3 atoms and molecules question answer are given below for practice. These practice questions are designed to test students understanding of the basic concepts.
Question 1. State the Law of Conservation of Mass.
Answer:
The Law of Conservation of Mass states that mass can neither be created nor destroyed in a chemical reaction. The total mass of reactants is equal to the total mass of products.
Question 2. What is the chemical formula of water and carbon dioxide? Name the elements present in them.
Answer:
Water $\left(\mathrm{H}_2 \mathrm{O}\right)$; Elements – Hydrogen and Oxygen
Carbon dioxide $\left(\mathrm{CO}_2\right)$; Elements – Carbon and Oxygen
Question 3. Define atomicity. Give the atomicity of oxygen $\left(\mathrm{O}_2\right)$ and phosphorus ($\mathrm{P}_4$).
Answer:
Atomicity is the number of atoms present in a molecule of an element.
Oxygen $\left(\mathrm{O}_2\right)$; Diatomic (Atomicity = 2)
Phosphorus ($\mathrm{P}_4$); Tetratomic (Atomicity = 4)
Question 4. Calculate the molecular mass of carbon dioxide ($\mathrm{CO}_2$).
(1) 44
(2) 45
(3) 43
(4) 42
Answer:
Atomic mass of C = 12 u, O = 16 u
Molecular mass of $\mathrm{CO}_2$ = 12 + (16 × 2) = 44 u
Hence, the correct answer is option (1).
Question 5. What is meant by a mole? How many particles are there in one mole of a substance?
Answer:
A mole is the amount of a substance that contains $6.022 \times 10^{23}$ particles. This number is called Avogadro’s constant.
Question 6: An atom of some element Y weighs $6.644 \times 10^{-23}$ g. Calculate the number of gram-atoms in 40 kg of it.
(1) 1200
(2) 100
(3) 1000
(4) 1100
Answer:
Mass of 1 mole Y atoms
= mass of 1 atom x Avogadro constant
=$6.644 \times 10^{-23} \times 6.022 \times 10^{23}$
= 40 g
So, the atomic mass of Y = 40
No. of gram-atoms (or moles) of X = (mass of y)/(atomic mass)
Number of gram atoms $=\frac{40 \times 1000}{40}$
Thus, the number of gram atoms = 1000 atoms
Hence, the answer is the option (3).
Question 7: How many atoms of hydrogen are there in 36 g of $\mathrm{NH}_4$?
(1) $9.635 \times 10^{23}$ atoms
(2) $96.35 \times 10^{24}$ atoms
(3) $4.81 \times 10^{23}$ atoms
(4) $48.17 \times 10^{23}$ atoms
Answer:
Molar mass (Molecular mass in gram) of $\mathrm{NH}_4$ = 14+4= 18 g
No. of moles of $\mathrm{NH}_4$ = 36/18 = 2 moles
Now, total moles of hydrogen atoms in one mole of $\mathrm{NH}_4$ = 4 moles
Thus, total Moles of Hydrogen Atoms in 2 moles of $\mathrm{NH}_4$ = 2 x 4 = 8 moles
Now, we know that in 1 mole of hydrogen atoms, the number of hydrogen atoms = $6.022 \times 10^{23}$
Thus, in 8 moles of hydrogen atoms, the number of atoms present = $8 \times 6.022 \times 10^{23}$
= $48.17 \times 10^{23}$ atoms
Hence, the answer is the option (4).
Question 8: Calculate the molar mass of CaCOH.
(1) 67g
(2) 72g
(3) 69g
(4) 70g
Answer:
The atomic mass of the consisting elements are:
Ca = 40u
C = 12u
O = 16u
H = 1u
Now, the molecular mass of CaCOH is given as:
Molecular mass = 40 + 12 + 16 + 1
Molecular mass = 69u
Thus, the molar mass of CaCOH = 69g
Hence, the answer is the option (3).
Approach to Solve Questions of Class 9 Science Chapter 3
Students can follow the approaches given below to solve atoms and molecules class 9 question answer. Referring to these NCERT Solutions for Class 9 will also help in understanding the correct methods. Follow the points below to make a good strategy for solving questions from this chapter:
1. Understand the structure of this chapters and break the chapter into manageable sections.
2. After that, try to learn and memorize key concepts of the chapter, such as Dalton’s Atomic Theory, the Law of Conservation of Mass, features of atomic and molecular masses, the mole concept, and molar mass, etc.
3. Write all the definitions on a separate sheet to solve definition based questions. Make a formula sheet related to the mole concept and molar masses to effectively solve numerical. You can also learn the topics from atoms and molecules class 9 notes available on our website.
4. Solve the NCERT examples and practice with in-text questions. Also, use some common tricks, like unit conversion, memorize symbols, and valency. Practice as many questions as asked in previous board exams and solve mock tests. Follow the class 9 science chapter 3 atoms and molecules solutions to learn more.
Topics and Subtopics Covered in the NCERT Textbook
All the topics and subtopics covered in atoms and molecules class 9 question answer are listed below. This helps students to get a clear overview of the chapter and focus on each concept clearly:
3.1 Laws of Chemical Combination
3.1.1 Law of Conservation of Mass
3.1.2 Law of Constant Proportions.
3.2 What is an Atom?
3.2.1 What Are the Modern-Day Symbols of Atoms of Different Elements?
3.2.2 Atomic Mass
3.2.3 How do Atoms exist
3.3 What is a Molecule?
3.3.1 Molecules of Elements
3.3.2 Molecules of Compounds
3.3.3 What is An Ion?
3.4 Writing Chemical Formulae
3.4.1 Formulae of Simple Compounds
3.5 Molecular Mass
3.5.1 Formula Unit Mass
Importance of Class 9 Science Chapter 3 Atoms and Molecules Solution
The class 9 science atoms and molecules question answer help students understanding concepts and improve their problem solving skills. Given below are some points on why these solutions are important:
1. The solutions explain concepts like atoms, molecules, laws of chemical combination, and atomic mass.
2. It helps to develop a base for topics like chemical reactions and equations in higher classes.
3. These atoms and molecules class 9 question answer help students in solving mole concept and mass calculation questions accurately.
4. They provide detailed solutions that help students in writing precise answers.