NCERT Solutions For Class 9 Science Chapter 3

NCERT Solutions Class 9 Science Atoms and Molecules

NCERT solutions for class 9 Science Chapter 3 Atoms and Molecules are prepared by our team of subject experts under the guidance of CBSE board. Our website BYJU’S provides chapter wise NCERT solutions for class 9 to assist students in their exam preparation and to gain more interest towards the topic. NCERT solutions for class 9 Science chapter 3 are available in PDF format and students can download these materials and use for their studies. NCERT solutions for class 9 Science chapter 3 pdf includes complete solutions present in the class 9 Science NCERT textbooks and are explained in a lucid manner along with the equations, formulas, diagrams and important questions.

General Question: Illustrate the relationship between a mole, Avogadro’s number and Mass.



Q1. A reaction is taking place wherein, 5g of sodium carbonate reacts with 7.2 gm of ethanoic acid. The products are 4.2g carbon dioxide, 3g water and 5g sodium ethanoate. Prove how these observations are in coherence with the law of mass conservation.


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

5g                                7.2g                    3g             4.2g                       5g

LHS    = RHS

12.2g = 12.2g

This observation thus shows that during a chemical reaction, mass of reactant = mass of product.


Q2.Hydrogen reacts with oxygen in the ratio 1:8 by mass to form water. How much oxygen is required to completely react with 4g of hydrogen?


We know hydrogen and water mix in the ratio 1: 8.

Thus, 1: 8 = 4: x           (where x is the amount of oxygen)

x      =8 * 4

= 32g

Therefore, 32g of oxygen would be required to completed react with 4g of hydrogen.


Q3. Which postulate of Dalton’s Atomic theory is the result of the law of conservation of mass?


The postulate of Dalton’s Atomic theory which is a result of the law of conservation of mass is, “Atoms can neither be created nor destroyed”.


Q4. Which postulate of Dalton’s Atomic theory can explain the law of definite proportions?


The postulate which can explain the law of definite proportions is that the relative number and the kinds of atoms are constant in a given compound.


Q5. What is an atomic mass unit?


An atomic mass unit is a unit of mass used to express weights of atoms and molecules where one atomic mass is equal to 1/12th the mass of one carbon-12 atom.


Q6. Why aren’t atoms visible to a naked eye?


Firstly, atoms are very very minute, measuring in nanometers. Secondly, except for atoms of noble gasses, they do not exist independently. For these basic reasons, we cannot see an atom with our naked eye.


Q7.Write the formulae for the following:

  • Carbon dioxide
  • Calcium hydroxide
  • Ammonium chloride
  • Aluminum chloride


Carbon dioxide-CO2, Calcium hydroxide- Ca(OH)2, Ammonium Chloride – NH4CL, Aluminum Chloride – AlCl3


Q8.What does chemical formula mean?


The symbolic representation of a chemical compound is called its chemical formulae. E.g. chemical formulae of salt is NaCl.


Q9. Find the number of atoms the following possess: – (i)H2O molecule (ii)H2S molecule


H2O3 atoms present

H2S  3 atoms present.


Q10.Find out the molecular masses ofCO2, O2, H2, CL2, NH3, C2 H2, CH3 OH.


H2 = 1 x 2 = 2u

O2= 16 x 2 =32U

CO2=1 x 12 + 2 x 16 =44U

CL2= 35.5 x 2= 71u

NH3 = 1 x 14 + 3 x 1 =17u

C2H2 = 2 x 12 + 2 x 1 =26u
CH3OH =12 + 3 x 1 + 16 + 1 = 32u


Q11.Work out the formula unit masses of K2CO3, ZnO, Na2O.

(Atomic masses of Zn=65u, Na= 23u, K=39u, C=12u and O= 16u)


ZnO= 65u + 16u = 81u

Na2O= (23u x 2) + 16u = 46u + 16u = 62u

K2CO3 = (39u x 2) + 12u + 16u x 3=138u


Q12. What is the mass of 1 carbon atom, if one mole of carbon atoms weigh 12gm?


1 mole of carbon atoms, 6.022 x 1023 = 12g

Therefore, mass of 1 carbon atom = 12/6.022 x 1023

= 1.99 x 10-23g



Q13.Out of 100g of sodium and 100g of iron which one is heavier?

(Atomic mass of Na=23u, Fe= 56u)

  1. 23g of Na = 6.022 x 1023atoms

100g   Na  = x1023 = 2.6182 x 1024atoms

56g of Fe  = 6.022 x 1023 atoms

100g of Fe=100 x (6.022/56) x 1023= 1.057 x 1024atoms

Thus, it is evident from the above calculations that 100g of Na has more atoms.


Q14. A 0.20g compound of oxygen and boron was found to contain 0.080g of boron and 0.12.Calculate the percentage composition of the compound by weight.


Compound Boron + Oxygen 

0.20               0.080      0.12

Percentage composition of boron:

0.20g 0.040g

100 x (0.080/0.20)=40%


Percentage composition of oxygen:

100 x (0.12/0.20)= 60%


Q15. 3g of carbon is burnt in 8g of oxygen, 11g of carbon dioxide is the by-product. What mass of CO2 is formed when 3g of carbon is burnt in 40g of oxygen?  Which law of chemical combination does the answer follow?


The chemical reaction of carbon burning in oxygen is:

C          +          O         CO2

1 mole             1 mole         44g

12g                 32g

It is given that 3g of carbon requires 8g gram of oxygen to produce 11g of carbon-dioxide. Thus when 3g of carbon burns in 40g of oxygen, we still get 11g of carbon dioxide with 40-8= 32g of oxygen remaining. The answer follows the law of constant proportion.


Q16.What do you understand by polyatomic ions?


Polyatomic ions are ions that contain more than one atom but they behave as a single unit e.g. CO3 2- , H2 PO4.


Q17. What is the chemical formula of the following compounds?


Vinegar, Sodium Thiosulphate

 Calcium Carbonate

Potassium Hydroxide

 Magnesium chloride.


Methane- CH4

Vinegar –CH3COOH

Sodium Thiosulphate – Na2S2O3.5H2O

Calcium Carbonate – CaCO3

Potassium Hydroxide – KOH

Magnesium Chloride –MgCl2


Q18. What are the names of the elements present in the following compounds:

Potash Alum, Quick lime, Plaster of Paris, Common Salt and Baking powder


Potash alum – Potassium Aluminum Sulphate

Elements –Potassium,   Aluminum, Sulphur, Oxygen and Hydrogen

Quick lime     – Calcium oxide

Elements- Calcium and oxygen


Plaster of Paris – Calcium Sulphate

Elements – Calcium, Sulphur, Oxygen and Hydrogen


Common Salt   – Sodium Chloride

Elements –Sodium and chlorine

Baking Powder – Sodium Hydrogen Carbonate

Elements- Sodium, Hydrogen, Carbon and Oxygen.


Q19. Find the molar mass of the following compounds:

Hydrochloric Acid (Hcl)


 Nitric Acid (HNO3)

 Acetic Acid (CH3COOH)



HCL = 1 + 35.5 = 36.5g

NH3= 14 + 1 x 3 =17g

HNO3 = 1 + 14 + 16 x 3 = 63g

CH3COOH = 12 + 3 x 1 + 12 + 16 x 2  + 1 =60g

C3H6O = 12 x 3 + 6 x 1 + 16 = 58g


Q20. Find the mass of:

  • 1 mole of oxygen
  • 5 moles of chlorine atoms
  • 10 moles of ammonia


1 mole of oxygen = 16g

5 mole of chlorine atoms = 5 x 35.5= 177.5g


1 mole of ammonia(NH3) = 14 + 3 x 1 =17g

Thus, 10 moles of ammonia = 10 x 17 = 170g.


Q21. Convert the given mass into mole:

  1. 10g of chlorine gas.
  2. 12g of Carbon dioxide.
  3. 10 gram of oxygen gas.


a) Given mass of chlorine gas = 10g.

Molar mass of chlorine gas = 71g.

Thus, mole of 10g of chlorine gas = 10/71=0.1408 moles.

b) Given mass of CO2 = 12g.

Molar mass of CO2 = 12 + 16 x 2 =44g.

Thus, mole 12g of CO2 = 12/44 =0.272 moles.

c) Given mass of O2 =10g.

Molar mass of O2 = 16 x 2 =32g.

Thus,  mole of 10g of O2 = 10/32 =0.3125 moles.


Q22. Find the mass of:

a) 2 moles of water molecules.

b) 5 moles of carbon dioxide molecules.


a) Mole of water molecules = 0.2

Molar mass of H2O =1 x 2 + 16 =18g

Mass of 0.2 moles of H2O =18 x 0.2 =3.6g

b) Mole of CO2 molecule = 0.5.

Molar mass of CO2 =12 + 16 x 2 =44g.

Mass of 0.5 moles of CO2 = 0.5 x 44 =22g.


Q23. What is the number of molecules of Sulphur (S8) present in 32g of solid sulphur?


Molar mass of sulfur = 256g = 6.022 x 1023molecule

Given mass of sulfur = 32g

Therefore, the number of molecules =  * 1023

=7.75 x 1022molecules.


Q24.Find the number of aluminum ions present in 0.046g of aluminum oxide. (For an   element, the mass of its ion and  atom  are the same)


Molar mass of aluminum oxide, Al2O3 = (2 x 27) + (3 x 16) = 102g


102g of Al2O3 contains = 2 x 6.022 x 1023 aluminum ions


0.046 g Al2O3 contains = * 1023

= 5.432 x 1020 Al3+ ions.


Q25. Multiple choice questions:

Choose the option:

1. The atomicity H2SO4 is:

(a)12    (b)7

(c)19   (d)8

2. The chemical formula for  acetic acid is:

 (a)CH3COOH   (b)CaCl2

(c)CaO             (d)CaCO3

3. The symbol for gold is:

(a)Cd               (b)Hg

(c)Gd               (d)Au

4. Noble gas molecules have:

(a) diatomic    (b)triatomic

(c) monoatomic (d)none of the above

5. The valency of oxygen in a water molecule is:

(a)1                 (b)2

(c)3                 (d)5

6. The molar mass of Ethyne, C2H2 is:

(a) 26g             (b) 21g

(c) 25g             (d) 15g           

7. How many moles of oxygen are present in 3.2g of oxygen atoms?

(a) 0.2moles                (b) 0.9 moles

(c) 1.2 moles               (d) 1 moles.

8.Which among the following is not proposed by Dalton in his atomic theory?

(a) Atoms cannot be divided further

 (b)Only atoms of the same elements can combine to form compounds.

(c) Atoms of different elements have different sizes, masses and chemical properties.

(d) Atoms can neither be created nor destroyed.

9.Pick the wrong one out:

(a) 1 mole of hydrogen = 1g

(b)1 mole of oxygen      = 32g

(c) 1 mole of Carbon       =18g

(d)1 mole of chlorine = 35.5g

 10. From the following compounds which one has a molecular mass of 106.

(a) Na2CO3      (b)H2SO4

(c)CaCl            (d)none of the above


Ans. 1-(b) , 2- (a) , 3- (d) , 4 –(c) , 5- (b) , 6- (a), 7 –(a) , 8 – (b) ,9- (c) , 10- (a). 


Q26.State the law of mass conservation.


According to this law, mass can neither be created nor destroyed in a chemical reaction or a physical transformation.


Q27.What do you understand by the law of constant proportion?


According to the Law of constant proportion, a chemical compound always has its compounding elements in definite proportion by mass, irrespective of the source and the type of chemical reaction.


Q28.Who was responsible for coining the term atom?


In modern science, John Dalton widely used it and he can be said to be responsible for coining it, but a more factually correct answer would be a Greek philosopher who goes by the name of Lucretius.


Q29.Define atom.


Atom is the smallest particle of matter which can take part in a chemical reaction.


Q30.What is a molecule?


A molecule is a group of atoms bonded together which represents the most fundamental unit of a chemical compound capable of taking part in a chemical reaction.


Q31.What is atomicity?


Atomicity is the number of atoms in a molecule.


Q32.Define atomic mass unit.


The Atomic mass unit is the sum of the atomic masses of all the atoms in a molecule.


Q32.What is the atomicity of oxygen and phosphorous?


Atomicity of oxygen, O2= 2 and atomicity of phosphorous, P4= 4.


Q33. Define ion.


An ion is an atom or a molecule with an electric charge (+ve or -ve), caused by gain or loss of a single or multiple electrons.


Q34. Provide a difference between cations and anions, along with an example for each.


Cations are positively charged ions, e.g.  Fe2+ whereas, anions are negatively charged ions, e.g. F


Q35.Define Avogadro’s constant.


Avogadro’s Constant (6.022 x 1023) is the number of atoms of an element present in one mole of that element.


Q36.Calculate the molecular mass of glucose, C 6H12 O6.


Molecular mass of C6H12 O6 = (12 x 6) + 12 + (16 x 6)

= 180u.


Q37.What is the unit to measure the size of an atom, and what is the size of a hydrogen atom?


An atom’s size is measured in nanometers and a hydrogen atom is 0.1 nm.


Q38. What does IUPAC stand for? State any one of its roles.


IUPAC stands for the International Union of Pure and Applied Chemistry. One of its roles is to approve the naming of elements


Q39. What is the Latin name for the following elements: gold, silver, sodium, potassium?


Gold – Aurum, Silver- Argentum, Sodium – Natrium, Potassium – Kalium.


Q40. What is the ratio by mass of the constituent elements in CO2, NH3 and H2O?


CO2 by mass of combining elements 12:32 3:8 (C:O)

NH3 by mass of combining elements  14:314:3 (N:H)

H2O by mass of combining elements   2:16  1:8 (H:O)


Q41.What is valency? Give the valency of the following elements:  beryllium, neon, magnesium, hydrogen, chlorine.


Valency is the number of electrons an atom can use to combine with other atoms or in other words, it is the combining power of an atom.

Beryllium-2, neon-0, magnesium-2, hydrogen-1, chlorine-2.


Q42.What is a polyatomic ion?


Polyatomic ions are composed of two or more atoms acting as a single unit. E.g. NH4+


Q43.What are the chemical formulae for: Acetone, copper nitrate and aluminum hydroxide.


Acetone- (CH3)2CO

Copper Nitrate-  Cu(NO3)

Aluminum Hydroxide – Al(OH)3


Q44.Calculate the number of moles in:

(i)36g of H2O

(ii)69g of Na


(i) Molar mass of H2O=2 x 1 + 16 =18g

Given mass of H2O =36g

Therefore, number of moles in 36g of H2O = 36/18 =2 moles

(ii) Molar mass of Na = 23g

Given mass of Na =69g

Therefore, number of moles in 69g of Na = 69/23 =3 moles.


Q45.What are the rules for writing the symbol of an element?


Firstly, the symbol has to be IUPAC approved.

The symbols are the first one or two letters of that element in English, Latin, German or Greek. E.g. He for helium, O2 for oxygen.

And, the first letter of the symbol has to be in upper case and the second letter has to be in lower case e.g. Na , Cl, etc.


Q46. What do you understand from relative atomic and relative molecular mass?

Ans.Relative atomic mass is the ratio of the mass of one atom to the 1/12th mass of a carbon-12 atom.

Relative molecular mass is the ratio of the mass of one molecule of an element to 1/12th of the mass of a carbon-12 atom.


Q47. H2O is the formula for water. What information do you get from this formula?


  • H2O represents water
  • H2O is a single molecule of water
  • H2O is a single mole of water. Thus, it contains 6.022 x 1023 molecules of water.
  • H2O contains 2 atoms of hydrogen and 1 atom of oxygen.
  • H2O has a molar mass of 18g


Q48.Differentiate between an atom and an Ion.


Atoms Ions
Neutral charge Positively or Negatively charged


Number of electrons and protons is equal  Number of protons and electrons isn’t equal



Q48.What is the formula unit mass of CaCl2 and NaCl.

(Na = 23, Cl=35.5, Ca=40)


Formula Unit Mass of NaCl = 23 + 35.5 = 58.5u

Formula Unit Mass of CaCl2= 40+(2 x 35.5)= 111u.


Q49. The ratio by mass of hydrogen to oxygen in a water molecule is 1: 8. Calculate the ratio of the number of atoms in a water molecule.


Element Ratio by mass Atomic mass Mass Ratio Simplest ratio
Atomic mass
H 1 1 1/1 =1 2
O 8 16 8/16=1/2 1

Thus, the ratio of the number of atoms in a water molecule is  H: O = 2: 1.


Q50. Write the chemical formula for the following compounds:

(a) Zinc carbonate

(b) Copper phosphate

(c) Aluminum carbonate

(d) Aluminum hydroxide

(e) Magnesium bicarbonate

(f) Calcium sulphide


(a)Zinc             Carbonate

Zn2+             CO32-

Formula = ZnCO3

(b)Copper        Phosphate

           Cu2+                                     PO43-

Formula = Cu3(PO4)2

(c)Aluminum         Carbonate

Al3+                                                           CO32-

                   Formula= Al2(CO3)3

(d)Aluminum            Hydroxide

Al3+                                     OH

                                    Formula =  Al(OH)3

(e)Magnesium           Bicarbonate

Mg2+                                HCO3

                                    Formula = Mg(HCO3)2

(f)Calcium                         Sulphide

Ca2+                                     S2-

   Formula = CaS


Q51.Write the atomicity of the following compounds:

  1. Al2(SO4)3
  2. Mg(HCO3)2
  3. ZnCO3
  4. HCL
  5. H2O
  6. H2SO4


  1. Al2(SO4)3 = 17
  2. Mg(HCO3)2 = 11
  3. ZnCO3 = 5
  4. HCL=2
  5. H2O = 3
  6. H2SO4=7


Q52. What is the difference between  2O, O2 and O3.


2O represents 2 atoms of oxygen, and it is not possible for it to exist independently.

O2 represents an oxygen molecule which has two constituent oxygen atoms.

O3 represents a single ozone molecule and it does exist independently.


Q53. (a)Explain how atoms exist.

          (b)What do you understand by atomicity?

          (c) Explain polyatomic ions.


(a) Atoms of most elements don’t exist independently they exist as molecules, for molecules are more stable. However, atoms of inert gasses are chemically unreactive and they exist independently. E.g. helium.

(b) Atomicity is the number of atoms in a molecule. E.g. The atomicity of H2O = 3.

(c) A polyatomic ion is an ion composed of multiple atoms acting as a single charged unit.


Q54. Find out

(1) the mass of a single oxygen atom

(2) the mass of a single oxygen molecule

 (3) the mass of a mole of oxygen gas

(4) the mass of an oxygen ion

(5) the number of atoms in a mole of an oxygen molecule.


(1) Mass of a single oxygen atom

1 mole of oxygen atom = 16gm =6.022 x 1023 atoms

Therefore, Mass of one oxygen atom = 16/6.022 x 1023 = 2.65 x 10-23 gm

(2) Mass of a single oxygen atom

1 molecule oxygen = O2 =2 x 16 = 32u

(3) Mass of a mole of oxygen gas

1 mole of oxygen = O2 = 2 x 16 =32u

(4) Mass of an oxygen ion = mass of an oxygen atom (since electrons have negligible mass)

(5) Number of atoms in a mole of oxygen molecule

We know, 1 mole of oxygen molecule, O2 = 6.022 x 1023 molecules.

1 molecule of O2= 2 atoms

Therefore in a mole of O2 , there are                       =6.022 x 1023 x 2 atoms

= 1.022 x 1024 atoms.


Q55. Explain atomic mass and gram atomic mass. Why does mass have different expressions viz,   ‘u’ and ‘gm’?


Atomic mass is the unit in which the mass of an atom is expressed, where one atomic mass unit is 1/12th the mass of a carbon-12 atom.

Gram atomic mass is the atomic mass of an element expressed in grams.

The mass of an atom or a molecule is expressed in ‘u’, whereas, the molar mass is expressed in ‘gm’.


Q56. Define a mole. Give the importance of the mole.


One mole of atoms, molecules, or particles is that amount of the particle(atoms, molecules and ions) whose mass is equal to that particle’s atomic or molecular mass in grams. 1 mole = 6.022 x 1023 particles of that substance.

Importance of a mole:-

  • Atoms and molecules are very small. So it gets bothersome weighing them in grams and trying to count them. Mole concept, however, allows us to count atoms and molecules by weighing macroscopic amounts of materials.
  • It gives us a universally accepted standard of mass.
  • It provides a standard for reaction stoichiometry.


Q57. A gold ring has 90% gold and 10% copper.

(a) How many atoms are there in a gram of gold

(b) What is the ratio of gold to copper in this jewelry?


(a) 1 gram of gold contains 90/100 = 0.9 grams of gold

Now, number of moles of gold  = Mass of gold/Atomic mass of gold

= 0.9/197 =0.0046

Therefore,0.0046 moles of gold will contain= 0.0046 x 6.022 x 1023

                                                                                                                                                   = 2.77 X 1021 atoms.

(b)Ratio of gold : copper = 9 : 1