Class 9 Science Chapter 3 – Atoms and Molecules explains the basic concepts of atoms, molecules, atomic mass, molecular mass, chemical formulas, and the laws of chemical combination. In this post, you will find the complete NCERT exercise questions and answers in simple and easy language. These Class 9 Science Chapter 3 solutions will help you understand the concepts clearly and prepare effectively for your exams.
ATOMS AND MOLECULES
EXERCISE
QUESTION-ANSWER
Ques 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 .
Mass of boron in the given sample compound = 0.096g
Mass of oxygen in the given sample compound = 0.144g
% composition of compound = % of boron and % of oxygen
0.096
= ———— x 100
0.24
96 x 100
= ————— x 100
24 x 1000
= 4 x 10
\ Percentage of boron = 40 %
144 x 100
= ————— x 100
24 x 1000
= 6 x 10 = 60
\ Percentage of oxygen = 60 %
OR
=
100% - 40%
\ Percentage of oxygen = 60 %
Ques
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.
Ans :- Carbon + Oxygen -----> Carbon dioxide
C + O2 ------------> CO2
Mass of Product Carbon dioxide (CO2)= 11g
Hence total mass of reactants = Total mass of Product
Therefore, the law of conservation of mass is proved.
Mass of Carbon = 3g ,
It is given that 3 g of carbon combine with 8 g of oxygen only.
This means that (50 - 8) = 42 g of oxygen will remain unreacted.
The remaining 42 g of oxygen will be left un-reactive.
In this case, too only 11 g of carbon dioxide will be formed
The above answer is governed by the law of constant proportions.
Ques 3:- What are polyatomic ions ? Give examples .
Ans :- A polyatomic ion is a
group of atoms carrying a charge. (positive or negative).
For example :- ammonium ion (NH₄)⁺,
hydroxide ion (OH)⁻,
Carbonate ion (CO₃)⁻,
Sulphate ion (SO₄)⁻.
Ques
4:- Write the chemical formula of the following :-
(a)
Magnesium chloride.
(b) Calcium
oxide .
(c) Copper nitrate.
(d)
Aluminium chloride .
(e) Calcium
carbonate.
(f) Hydrogen chloride
.
Ans (i):- Formula of Magnesium chloride:-
Formula :- MgCl2
Ans (ii):- Formula of Calcium Oxide :-
Formula :- Ca₂O₂ or CaO
Ans (iii):- Formula of Copper
nitrate :-
Formula :- CuNO₃
Ans (iv):- Formula of Aluminium chloride:-
Formula :- AlCl₃
Ans (v):- Formula of Calcium carbonate:-
Ques 5:- Give the names of the elements present in the following compounds :
(a) Quick lime
(b) Hydrogen bromide
(c) Baking powder
(d) Potassium sulphate
Ans:-
Ques 6:- Calculate the molar mass of the following substances :
(a)
Ethyne (C₂H₂) .
(b) Sulphur molecule (S₈) .
(c) Phosphorus molecule (P₄) ( atomic mass of phosphorus = 31 ) .
(d)
Hydrochloric acid (HCl)
(e) Nitric acid
(HNO₃) .
Ans:- Mass of 1 mole molecule of a substance is called
its molar mass.
(a) Atomic mass of Carbon =12u ,
Atomic mass of Hydrogen =1u
= 2 × 12 + 2 × 1 = 26u
\ Molar Mass of ethyne C2H2= 26 g.
(b)
Atomic mass of Sulphur =12u
Molar mass of Sulphur molecule (S₈) = 8 x Atomic mass of S
= 8 x 32
\ Molar mass of Sulphur molecule (S8) = 256 g
(c) Atomic mass of phosphorus molecule= 31u
Molar mass of phosphorus molecule(P4) = 4 x Atomic mass of P
= 4 x 31
\ Molar mass of phosphorus molecule (P4) = 124 g
(d) Atomic mass of hydrogen =1u ,
Atomic mass of hydrogen =35.5u
Molar
mass of Hydrochloric acid (HCl) = Atomic mass of H + Atomic mass of Cl
= 1 + 35.5
\ Molar mass of Hydrochloric acid (HCl) = 36.5 g
(e) Atomic mass of hydrogen =1u ,
Atomic mass of nitrogen =14u
Atomic mass of oxygen =16u
Molar mass of nitric acid (HNO3) = Atomic mass of H + Atomic mass of N + 3 x Atomic mass of O.
= 1 + 14 + 3 x 16
=> 1 + 14 + 48
\ Molar mass of Hydrochloric acid (HCl) = 63 g
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