Welcome to the Chapter 3 - Atoms and Molecules, Class 9 Science NCERT Solutions page. Here, we provide detailed question answers for Chapter 3 - Atoms and Molecules. The page is designed to help students gain a thorough understanding of the concepts related to natural resources, their classification, and sustainable development.
Our solutions explain each answer in a simple and comprehensive way, making it easier for students to grasp key topics Atoms and Molecules and excel in their exams. By going through these Atoms and Molecules question answers, you can strengthen your foundation and improve your performance in Class 9 Science. Whether you’re revising or preparing for tests, this chapter-wise guide will serve as an invaluable resource.
Law of conservation of mass: Mass can neither be created nor be destroyed in a chemical reaction.
Given reaction: sodium carbonate + ethanoic acid → sodium ethanoate + carbon dioxide + water
Total mass of reactants = 5.3 + 6 = 11.3 g.
Total mass of products = 8.2 + 2.2 + 0.9 = 11.3 g
Total mass of reactants = Total mass of products
Hence, these observation are in agreement with the law of conservation of mass.
Given,
Ratio of hydrogen and oxygen by mass = 1:8
Mass of oxygen gas that completely reacts with 1g of hydrogen gas is equal to 8 g i.e Mass of oxygen gas that completely reacts with 3 g of hydrogen gas
= 8 x 3 = 24g.
Hence, 24 g mass of oxygen gas is required to react completely with 3 g of hydrogen gas.
Postulate of Dalton's atomic theory: Atoms are indivisible particles which cannot be created or destroyed in a chemical reaction. It is the result of the law of conservation of mass.
The postulate of Dalton's theory, "The relative numbers and finds of atoms are content in a given compound” can explain the law of definite proportion.
Atomic mass unit: Mass unit equals to exactly 1/12 th the mass of one atom of carbon-12 is known as an atomic mass unit.
Note:
Carbon-12: Standard reference for measuring atomic Masses.
1 amu (atomic mass unit) = 1.66x10-24 g.
The size of an atom is very very small that we do not see with our naked eyes. Atoms of mait elemente cannot exist independently.
(i) Na2O (Sodium oxide)
(ii) AlCl2 (Aluminium chloride)
(iii) Na2S (Sodium sulphide)
(iv) Mg(OH)2 (Magnesium hyelsonide)
(i) Aluminium sulphate
(ii) Calcium chloride
(iii) Potassium sulphate
(iv) Potassium Nitrate
(v) Calcium Carbonate
It is the symbolic representation of the compound's composition. For example, the chemical formula of Calcium Carbonate is CaCo3.
(i) 3 atoms are present in the H2S molecule. There are 2 atoms of hydrogen and one is sulphur.
(ii) 5 atoms are present in PO43- ion. There are 4 atoms of oxygen and one in phosphorus.
Atomic mass includes the masses of three subatomic particles which make up an atom i.e. proton, neutron and electron.
Unit of atomic mass is 'u'
‘u’ = unified atomic mass unit & 1/12 th mass of single carbon-12 atom
‘u’ = 1.66 x 10-24 g
(Just for understanding - Atomic mass approximately equal to twice of atomic no.)
Molecular mass of a substance is the sum of atomic masses of all the atoms in a molecule of a subitance.
Unit of molecular mass = 'u’
Molecular mass of H2 = 2 x 1 = 2u (Atomic mass of H=1)
Molecular mass of O2 = 16 x 2 = 32 u (Atomic mass of O=16)
Molecular Mass of Cl2 = 35.2 x 2 = 71 u (Atomic mass of Cl = 35.5)
Molecular Mass of CO2 = 12 x 1 + 16 x 2
= 12 + 32 = 44 u ( Atomic Mass of C = 12)
Molecular Mass of CH4 = 12 x 1 + 4 x 1
= 12 + 4 = 16 u
Molecular Mass of C2H6 = 12 x 2 + 6 x 1
= 24 + 6 = 30 u
Molecular Mass of C2H4 = 12 x 2 + 4 x 1
= 24 + 4 = 28 u
Molecular Mass of NH3 = 14 X 1 + 3 x 1
= 14 x 3 = 17 u (Atomic Mass of N = 14)
Molecular Mass of CH3OH = 12 x 1 + 4 x 1 + 16 x 1
= 12 + 4 + 16 = 32 u
Formula unit mass of ZnO
= 1 x Atomic mass of Zn + 1 x Atomic mass of ‘O’ atom
= 1 x 65 + 1 x 16
= 65 + 16
= 81 u
Formula unit mass of Na2o
= 2 x Atomic mass of Na + 1 x Atomic mass of ‘O’ atom
= 2 x 23 + 1 x 16
= 46 + 16
= 62 u
Formula unit mass of K2CO3
= 2 x Atomic mass of K + 1 x Atomic mass of C + 3 x Atomic mass of ‘O’
= 2 x 39 + 1 x 12 + 3 x 16
= 78 + 12 + 48
= 138 u
1 mole of carbon atoms = 6.022 x 1023
1 mole of carbon atoms weighs = 12 gram
Mass of 6.022 x 1023 atoms of carbon = 12 gram
1 atom of carbon = 12 / 6.022 x 1023
= 1.99 x 10-23 gram
Hence, Mass of 1 atom of carbon
= 1.99 x 10-23 gram
Atomic mass of sodium = 23 u
Gram atomic mass of sodium = 23 g
1 mole or 23 g of sodium has = 6.022 x 1023 atom
100 g of sodium has = 6.022x1023 / 23 x 100
= 2.618 x 1024 atom
Atomic mass of Ison (fe) = 56 u
Gram atomic mass of fe = 56 g
1 mole of fe has = 6.022 x 1023 atom
56 g of fe = 6.022 x 1023 atom
100 g of fe = 6.022 x 1023 / 56 x 100
= 1.075 x 1024 atom
Hence, 100 gram of Na has more number of atoms than 100 gram of fe.
Mass of Boron = 0.096 g
Маss of Oxygen = 0.144 g
Mass of sample of compound of oxygen and Boron = 0.24 g
% of Boron by weight in the compound = 0.096 x 100 / 0.24 = 40%
% of Oxygen by weight in the compound = 0.144 x 100 / 0.24 = 60%
Molar mass of S8 = 8 x 32 = 256 g
256 g = 6.022 x 1023 no. of molecules of S8
No. of molecules present in 16 g of solid sulphur = 6.022x1023 / 256 x 16 = 0.376 x 1023
= 3.76x1022
Molar Mass of Al2O3 = 2 x Atomic mass of Al + 3 x Atomic mass of O
= 2 x 27 + 3 x 16
= 54 + 48 = 102 g
102 g of Al2O3 contain = 2 x 6.022 x 1023 aluminium ion
= 0.051 g of Al2O3 = 2 x 6.022 x 1023 / 102 x 0.051
= 6.022 x 1023 x 10-3 = 6.022 x 1020
1 mole of C = 12 g
1 mole of O2 = 32 g
C + O2 → CO2
12 g + 32 g → 44 g
4 4 4
[3g + 8g - 11g] → constant
8 g of O2 will be consumed from 50g of O2 and 42 g of O2 will be unreactive. It shows that 12 g of carbon burns in 32 g oxygеn from 44 g of CO2 bесоuse 3 g of carbon reacts with 8 g of oxygen to form 11 g of CO2. The same is given to us. Consequently 11.0 g of CO2 will be formed when 3.0 g of carbon is burnt in 50 g of oxygen consuming 8 g of O2, leaving behind 42 g of O2. The answer governs the Law of constant Proportion.
Polyatomic Ions are that ions which have more than one atom of same kind of different kind and behave as a single unit is known as polyatomic ion.
E.g. : OH- , SO42- , PO43-
(a) MgCl2
(b) Cao
(c) Cu(NO3)2
(d) AlCl3
(е) CаCо3
(a) Calcium oxide (Cao), There are two type of elements are present first in calcium and the other is oxygen
(b) Chemical formula of Hydrogen Bromide is HBr. There are two types of element present, the first one in hydrogen and the other in Bromine.
(c) Chemical formula of Baking powder is NaHCO3 (Sodium Hydrogen Carbonate). There are 4 types of element sodium, hydrogen, carbon and oxygen.
(d) Chemical formula of Potassium sulphate is K2SO4. There are three types of element: Potassium, sulphur and oxygen.
Molar mass: The mass of the mole of substance is known as molar mass.
Unit = gm/mol.
(a) C2H2 = H - C = C - H
= 2 x Atomic mass of C + 2 x Atomic mass of H
= 2 x 12 + 2 x 1
= 24 + 2 = 26 g
(b) S8 = 8 x atomic mass of sulphur.
= 8 x 32 = 256 g
(c) P4 = 4 x atomic mass of phosphorus
= 4 x 31 = 124 g
(d) HCl = 1 x atomic mass of H + 1 x atomic mass of Cl
= 1 x 1 + 1 x 35.5
= 1 + 35.5 = 36.5 g
(e) HNO3 = 1 x atomic mass of H + 1 x atomic mass of N + 3 x atomic mass of O
= 1 x 1 + 1 x 14 + 3 x 16
= 1 + 14 + 48 = 63 g
1 mole of any particle = Relative mass of that particle in grams
1 mole of any molecule = molecular mass in grams
(a) 1 mole of nitrogen atom = 14 grams
(b) 1 mole of aluminium atom = 27 grams
4 mole of aluminium atom = 27 x 4 = 108 grams
(c) Na2SO3 = 2 x atomic mass of Na + 1 x atomic mass of sulphur 3 x atomic mass of O
= 2 x 23 + 1 x 32 + 3 x 16
= 46 + 32 + 48 = 126 g
1 mole of Na2SO3 = 126 g
10 moles of Na2SO3 = 126 x 10 = 1260 grams
Mass (in grams) = no. of moles x molecular mass (g/mol)
(a) Molar mass of O2 = 2 x 16 = 32 g/mol
Mass = no. of mole x molar mass
12 = n x 32
n = 12/3 = 3/8
n = 0.375 mole
(b) Given mass = 20 g
Molar mass of H2O = 2 x 1 + 1 x 16
= 2 + 16 = 18 g/mol
Mass = no. of mole x molar mass
20 = n x 18
n = 20/18 = 10/9
n = 1.11 mole
(c) Given mass = 22g
Molar mass of CO2 = 1 x 12 + 2 X 16
= 12 + 32 = 44 g/mol
Mass = no. of moles x molar mass
22 = n x 44
n = 22/44 = ½
n = 0.5. mole
(a) No. of moles = 0.2
Molar mass of oxygen = 16 g/mol
Mass = no. of moles of molar mass
Mass = 0.2 x 16 = 3.2 g
(b) No. of moles = 0.5
Molar mass of H2O = 2 x 1 + 16
= 2 x 16 = 18 g/mole
Mast = no. of moles x molar mass
Mass = 0.5 x 18 = 9.0 g
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