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Everything in our environment, from the tiniest grain of sand to the vast universe, is made up of atoms and molecules. Atoms combine in various ways to form molecules that constitute all substances (solid, liquid, or gas). The information on the structure and behaviour of atoms and molecules helps us understand chemical reactions, the composition of materials, and as well as biological processes. NCERT Exemplar Class 9 Science Solutions Chapter 3 provides a clear and detailed understanding of these fundamental topics. Concepts like the laws of chemical combination, atomic mass, molecular mass, and chemical formulas are covered in this chapter, which will help students build a strong foundation in chemistry. This chapter not only develops a deeper understanding of chemistry but also helps students score well in exams by answering simple yet important questions.
The NCERT Exemplar solutions for this chapter aim at simplifying complex topics and helping students grasp the concepts of atomic structure and molecular formation. These solutions, which were meticulously created by knowledgeable educators, adhere to the CBSE-recommended syllabus.
Let us examine these solutions to get a better understanding of the topic “Atoms and Molecules”.
Question:1
Which of the following correctly represents 360 g of water?
(i) 2 moles of
(ii) 20 moles of water
(iii)
(iv)
(a) (i)
(b) (i) and (iv)
(c) (ii) and (iii)
(d) (ii) and (iv)
Answer: D
The average mass of one mole of
The molecular weight of water is 18 g/mol. Hence, one mole of water weighs 18 g.
One mole of water contains
2 moles of
Therefore, the mass of
Question:2
Which of the following statements is not true about an atom?
(a) Atoms are not able to exist independently
(b) Atoms are the basic units from which molecules and ions are formed
(c) Atoms are always neutral in nature
(d) Atoms aggregate in large numbers to form the matter that we can see, feel or touch
Answer: A
Atoms are not able to exist independently is not true, Atoms of most elements are not able to exist independently but inert gases can exists as atoms.
Question:3
The chemical symbol for nitrogen gas is
(a) Ni (b)
Answer: B
Chemical formula of Nitrogen is N but nitrogen exist as a molecule of two atoms. Therefore, the chemical symbol of nitrogen gas is
Question:4
The chemical symbol for sodium is
(a) So (b) Sd (c) NA (d) Na
Answer: D
Sodium word is derived from Latin word Natrium hence the chemical name of sodium is Na.
Question:5
Which of the following would weigh the highest?
(a) 0.2 mole of sucrose
(b) 2 moles of
(c) 2 moles of
(d) 10 moles of
Answer: C
From mole concept,
1 mole = molecular weight in gram.
Weight of a sample in gram = Number of moles × Molar mass
Mass of 1 mole of sucrose
Mass of 1 mole of
Mass of 1 mole of
Mass of 1 mole of
(a) 0.2 moles of
(b) 2 moles of
(c) 2 moles of
(d) 10 moles of
Question:6
Which of the following has maximum number of atoms?
(a) 18g of
(b) 18g of
(c) 18g of
(d) 18g of
Answer: D
(a) 18 g of water
(b) 18 g of oxygen =
(c) 18 g of
(d) 18 g of
Question:7
Which of the following contains maximum number of molecules?
(a) 1g
(b) 1g
(c) 1g
(d) 1g
Answer: C
Molar mass of other molecules are much higher than given mass, so number of molecules in them will be less than that in hydrogen.
1 g of
Question:8
Mass of one atom of oxygen is
(a)
(b)
(c)
(d) 8u
Answer:
Mass of one atom of oxygen
Question:9
3.42 g of sucrose are dissolved in 18g of water in a beaker. The number of oxygen atoms in the solution are
(a)
(b)
(c)
(d)
Answer: A
1 mol of sucrose
Here,
1 mol of sucrose
= 1mol
1mol of water
Total number of oxygen atoms = Number of oxygen atoms from sucrose + Number of oxygen atoms from water
Number of oxygen atoms in solution
Question:10
A change in the physical state can be brought about
(a) only when energy is given to the system
(b) only when energy is taken out from the system
(c) when energy is either given to, or taken out from the system
(d) without any energy change
Answer: C
A change in the physical state can be brought about when energy is either given to, or taken out from the system. When a solid change into liquid, it absorbs energy. It is because, energy change helps in changing the magnitude of attraction forces between the particles, thus helps in changing the physical states (i.e., solid, liquid, gas) of matter. When a liquid change into solid, it releases energy.
Question:11
Which of the following represents a correct chemical formula? Name it.
(a) CaCl
(b)
(c)
(d) NaS
Answer: B
The correct formula is CaCl2 (valency of Ca = 2, valency of Cl = 1).
The correct formula is
The correct formula is
Question:12
Write the molecular formulae for the following compounds
(a) Copper (II) bromide
(b) Aluminium (III) nitrate
(c) Calcium (II) phosphate
(d) Iron (III) sulphide
(e) Mercury (II) chloride
(f) Magnesium (II) acetate
Solution:
(a) Copper(II) Bromide –
(b) Aluminium(III) nitrate –
(c) Calcium (II) phosphate –
(d) Iron(III) sulphide –
(e) Mercury(II) chloride –
(f) Magnesium(II) acetate –
Question:14
Write the cations and anions present (if any) in the following compounds
(a)
(b)
(c)
(d)
Solution.
Anions Cations
(a)
(b)
(c) It is a covalent compound
(d)
Question:15
Give the formulae of the compounds formed from the following sets of elements
(a) Calcium and fluorine
(b) Hydrogen and Sulphur
(c) Nitrogen and hydrogen
(d) Carbon and chlorine
(e) Sodium and oxygen
(f) Carbon and oxygen
Answer:
(a) Calcium and fluoride - Calcium Fluoride
(b) Hydrogen and sulphur - Hydrogen Sulphide
(c) Nitrogen and hydrogen - Ammonia
(d) Carbon and chlorine - Carbon Tetrachloride
(e) Sodium and oxygen - Sodium Oxide
(f) Carbon and oxygen - Carbon dioxide
Question:16
Which of the following symbols of elements are incorrect? Give their correct symbols
(a) Cobalt CO
(b) Carbon c
(c) Aluminum AL
(d) Helium He
(e) Sodium So
Answer:
(a) Incorrect, the correct symbol of cobalt is Co
(b) Incorrect, the correct symbol of carbon is C
(c) Incorrect, the correct symbol of aluminum is Al
(d) Correct (He)
(e) Incorrect, the correct symbol of sodium is Na
Question:18
State the number of atoms present in each of the following chemical species
(a)
(b)
(c)
(d)
Solution.
(a)
(b)
(c)
(d) CO = 1C + 1(O) = 2
Question:19
What is the fraction of the mass of water due to neutrons?
Solution.
In water molecule
neutrons in H = 0)
Mass of 8 neutrons
Molar mass of water
There are 8 neutrons in one atom of oxygen
Mass of 8 neutrons
Fraction of mass of water due to neutrons =
Question:20
Does the solubility of a substance change with temperature? Explain with the help of an example.
Solution.
Yes, the solubility of a substance depends on its temperature. The solubility of solids in liquids usually increases on increasing the temperature and decreases on decreasing the temperature. The solubility of gases in liquids usually decreases on increasing the temperature and increases on decreasing the temperature.
For example, one can dissolve more sugar in hot water than in cold water.
Question:21
Classify each of the following on the basis of their atomicity.
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
(i) HCl
(j)
(k) He
(l) Ag
Answer:
(a) Diatomic (2 atoms)
(b) Triatomic (3 atoms)
(c) Triatomic (3 atoms)
(d) Polyatomic (8 atoms)
(e) Polyatomic (4 atoms)
(f) Polyatomic (4 atoms)
(g) Polyatomic (14 atoms)
(h) Triatomic (3 atoms)
(i) Diatomic (2 atoms)
(j) Polyatomic (5 atoms)
(k) Monoatomic (1 atom)
(l) Monoatomic (1 atom)
Question:22
Answer.
There are many ways by which we can differentiate between sugar and salt without testing. We can identify the powder by heating. The powder will char if it is a sugar. Dissolve salt and sugar separately in alcohol, sugar will be dissolved in it while salt will not be dissolved. the powder can be dissolved in water and checked for electrical conductivity. If it conducts, then the powder is salt.
Question:23
Calculate the number of moles of magnesium present in a magnesium ribbon weighing 12 g. Molar atomic mass of
magnesium is
Answer:
Weight of a sample in grant = Number of moles × Molar mass
Atomic mass of Mg
24 g of Mg = 1 mol
12 g of Mg = 12/24 = 0.5 mol
Question:24
Verify by calculating that
(a) 5 moles of
(b) 240 g of calcium and 240 g magnesium elements have a mole ratio of
Answer:
(a)
5 moles of
5 moles of
Hence, they do not have same mass.
(b) Number of moles in 240g Ca metal
Number of moles in 240g of Mg metal
Ratio
Question:25
Find the ratio by mass of the combining elements in the following compounds. (You may use Appendix-III)
(a)
(b)
(c)
Answer:
(a)
(b)
(c)
(d)
(e)
(f)
Question:26
Calcium chloride when dissolved in water dissociates into its ions according to the following equation.
Calculate the number of ions obtained from
Answer:
Total number of ions by 2 moles of
Number of ions = Number of moles of ions
Question:27
The difference in the mass of 100 moles each of sodium atoms and sodium ions is 5.48002 g. Compute the mass of an electron.
Answer:
Number of electrons in Na atom = 11
Number of electrons in
For 1 mole of Na atom and Na+ the difference in electrons = 1 mole
For 100 moles of Na atoms and Na+ ions the difference = 100 moles of electrons
Mass of 100 moles of electrons = 5.48002 g
Mass of 1 mole of electrons
Mass of one electron
Question:28
Answer:
Molar mass of HgS
Mass of Hg in 232.6g of HgS =200.6g
Mass of Hg in 225g of HgS
Question:29
One way of srews weigh
Mass of earth is
Question:30
1 mole of oxygen atoms
∴ Number of moles of oxygen atoms
Question:31
Raunak took 5 moles of carbon atoms in a container and Krish also took 5 moles of sodium atoms in another container of same weight. (a) Whose container is heavier? (b) Whose container has more number of atoms?
Solution.
(a) Mass of sodium atoms carried by Krish
While mass of carbon atom carried by Raunak
Thus, Krish’s container is heavy.
(b) As the two bags have the same number of moles of atoms, both the bags will have the same number of atoms.
Question:32
Question:33
The visible universe is estimated to contain
Answer:
1 mole of stars equals
Number of moles of stars
Number of moles of stars = 0.0166
Question:34
What is the SI prefix for each of the following multiples and submultiples of a unit?
(a)
(e)
Answer:
(a) kilo
(b) deci
(c) centri
(d) micro
(e) nano
(f) pico
Question:36
Compute the difference in masses of
Answer:
Mass of
Question:38
Compute the number of ions present in 5.85 g of sodium chloride.
Answer:
Or, 0.1 moles of NaCl particles.
Each NaCl particle is equivalent to one
Total moles of ions
No. of ions
Question:39
Answer:
One gram of gold sample will contain
Number of moles of gold
One mole of gold contains
Question:40
What are ionic and molecular compounds? Give examples.
Answer:
While forming some compounds, atoms may gain or lose electrons, thereby forming electrically charged particles called ions. Compounds that are formed by the attraction of cations and anions are known as ionic compounds.
Ex :
Compounds formed by the bonding of uncharged species are known as molecular compounds. The bonding is called covalent bonding. Molecular compounds are formed by sharing of electrons between the two atoms. Ex:
Question:41
Ionization of Al atom occurs as
Therefore,
It is because 1 mole of Al atom have
Question:42
A silver ornament of mass’m’ gram is polished with gold equivalent to 1% of the mass of silver. Compute the ratio of the number of atoms of gold and silver in the ornament.
Answer:
Mass of silver =mg
Mass of gold deposited
No. of atoms of silver
No. of atoms of gold
Ratio of the number of atoms of gold to silver
Question:43
Therefore, number of molecules of
Question:44
(a) In a chemical reaction, the sum of the masses of the reactants and products remains unchanged. This is called Law of conservation of mass.
It states that mass is neither created nor destroyed in chemical reactions.
(b) A group of atoms carrying a fixed charge on them is called ions.
An ion is a charged atom or molecule. It is charged because the number of electrons are not equal to the number of protons in the atom or molecule.
(c) The formula unit mass of
(d) Formula of sodium carbonate is
Question:46
(a) In this crossword puzzle (Fig 3.2), names of 11 elements are hidden. Symbols of these are given below. Complete the puzzle.
1. Cl 7. He
2. H 8. F
3. Ar 9. Kr
4. O 10. Rn
5. Xe 11. Ne
6. N
(b) Identify the total number of inert gases, their names and symbols from this cross word puzzle.
Answer:
(a)
(b) Six inert gases:
Helium (He); Neon ( Ne); Argon (Ar); Krypton (Kr); Xenon (Xe); Radon (Rn).
Question:48
The chemical reaction involved is :
To produce 18g of glucose, mass of water consumed =
To produce 18g of glucose, mass of water consumed
Question: An atom of some element Y weighs 6.644×10−23 g. Calculate the number of gram-atoms in 40 kg of it.
(1) 1200
(2) 100
(3) 1000
(4) 1100
Solution: Mass of 1 mole Y atoms
= mass of 1 atom x Avogadro constant
=6.644 x 10−23 x 6.022 x 1023
= 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
Thus, the number of gram atoms = 1000 atoms
Hence, the correct answer is option (3).
Question: Calculate the molar mass of KAl(SO4)2·12H2O.
(1) 462g
(2) 480g
(3) 470g
(4) 472g
Solution: The atomic mass of the consisting elements are given below:
K = 39u
Al = 27u
S = 32u
O = 16u
H = 1u
Now, the molecular mass of KAl(SO4)2·12H2O is given as:
Molecular mass = 39 + 27 + 32x2 + 16x8 + 12x1 + 12x16
Molecular mass = 462u
Thus, the molar mass of KAl(SO4)2·12H2O = 462g
Hence, the answer is the option (1).
Approach is crucial to solving the questions effectively. The following are some points that can help you build a good strategy:
1). Firstly, master the key concepts like:
2). Remember all the Symbols, Formulas, and Valencies:
3). Students must focus on the mole concept:
4). Before solving questions, confirm which question type is asked and solve accordingly.
5). Practice numerical concepts again and again for a better understanding of the concepts.
6). Solve questions
NCERT exemplar Class 9 Science solutions chapter 3 includes the following topics :
An atom is the smallest unit of an element that can take part in a chemical reaction. A molecule is formed when two or more atoms are chemically bonded together.
If you react 4g of Hydrogen with 32g of Oxygen to produce water, you will get exactly 36g of water. The mass of Hydrogen + Oxygen (reactants) equals the mass of Water (product).
A chemical formula is a symbolic representation of a molecule that shows the types of atoms present and the number of atoms of each type. For example, H₂O is the chemical formula for water, indicating two hydrogen atoms and one oxygen atom.
One atomic mass unit (amu or u) is defined as 1/12th the mass of a carbon-12 atom.
Avogadro's number (NA) is the number of entities in one mole, which is approximately 6.022 x 10²³.
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