This post provides you a complete solution of intext and End exercise questions. This chapter provides information about atoms and molecule and you will learn structure of atom in the next chapter.
.png "RBSE Class 9 Science Chapter 3 Atoms and Molecules Solutions")
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Chapter
3Atoms and Molecules
Intext Questions
Answers of Questions on Page 27 -28
Q.1 In a reaction 5.3
of sodium carbonate reacted with 6 g of ethanoic acid. The products were 2.2 g
of carbon dioxide, 0.9 g water and 8.2 g of sodium ethanote. Show that these
observations are in agreement with the law of conservation of mass.
Answer
Q.2 Hydrogen and oxygen
combine in the ration of 1 : 8 by mass to form water. What mass of oxygen gas
would be required to react completely with 3 g of hydrogen gas?
Answer
Q.3 Which postulate
Dalton’s atomic theory is the result of the law of conservation of mass?
Answer
In every chemical reaction, total
masses of all the reactants, is equal to the masses of all of the products.
Atoms cannot be created nor destroyed in a chemical reaction.
Q.4 Which postulate of
Dalton’s atomic theory can explain the law of definite proportions?
Answer
In a given chemical compound, the
proportions by mass of the elements that compose it are constant.
Answers of Questions on Page 30
Q.1 Define the atomic
mass unit.
Answer
One atomic mass unit is equal to
exactly one twelfth (1/12)th the mass of one atom of C-12.
Q.2 Why is it not
possible to see an atom with naked eyes?
Answer
Atoms are very small to be seen with
naked eyes. Their size is measured in nanometers.
(1 nm = 10-9 m).
Answers of Questions on Page 34
Q.1 Write down the
formulae of:
(i) Sodium oxide (ii) Aluminium chloride
(iii) Sodium sulphide (iv) Magnesium
hydroxide
Answer
Q.2 Write down the
names of compounds represented by the following formulae:
(i) Al2(SO4)3 (ii) CaCl2 (iii) K2SO4 (iv) KNO3
(v) CaCO3
Answer
(i) Aluminium Sulphate
(ii) Calcium
Chloride
(iii) Potassium Sulphate
(iv) Potassium Nitrate
(v) Calcium Carbonate
Q.3 What is meant by
the term chemical formula?
Answer
The chemical formula of the compound is
symbolic representation of its composition. Eg- Chemical formula of Sodium
oxide is Na2O
Q.4 How many atoms are
present in a
(i) H2S molecule and (ii) PO4-3 ion ?
Answer
(i) H2S → 3atoms are present
(ii) PO4-3 → 5 atoms are
present
Answers of Questions on Page 35
Q.1 Calculate the
molecular masses of H2, O2 , Cl2 ,CO2,
CH4 ,C2H6 , C2H4 , NH3
,CH3OH
Answer
Answer
Answers of End Exercise Questions
Q.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
mass.
Answer
Percentage of Boron = `\frac\text{mass of Boron}{Mass of compounds}\times 100`
Percentage of oxygen=`\frac\text{mass of oxygen}{Mass of compounds}\times 100`
=`\frac{0.144}{0.24}\times100 =60.0`
Hence, the mass percentage of boron and oxygen
in the given compound are 40% and 60% respectively.
Q.2 When 3.0 g of
carbon is burnt in 8.0 g of oxygen, 11.0 g of carbon dioxide is produced. What
mass of carbon dioxide will be formed when 3.0 g of carbon is burnt in 50.0 g
of oxygen? Which law of chemical combination will govern your answer?
Answer
For first case:
3.0
g of carbon is burnt in 8.0 g of oxygen to form 11.0 g of CO2.
For
second case:
3.0
g of carbon must also combine with 8.0 g of oxygen only. This means that (50-8)
= 42 g of oxygen will remain un-reacted.
The
mass of CO2 in this case must also be 11 g. So, law of constant
proportion is also valid here.
Q.3 What are polyatomic
ions? Give examples.
Answer
The ions which contain more than one
atoms and behave as a single unit are called polyatomic ion e.g., PO42-, CO35-,
SO42- , OH-
etc.
Q.4 Write the chemical
formulae of the following:
(a) Magnesium chloride
(b) Calcium
oxide
(c) Copper nitrate
(d) Aluminium
chloride
Answer
Q.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
(a) Quick
lime is calcium oxide
Elements – Calcium and Oxygen
(b). Hydrogen bromide
Elements – Hydrogen and
bromine
( c). Baking powder is Sodium hydrogen
carbonate
Elements – Sodium,
hydrogen, carbon and oxygen
(d). Potassium sulphate
Elements – Potassium, suphur,
oxygen
Q.6 Calculate the molar
mass of the following substances:
(a) Ethyene, C2H2
(b) Sulphur molecule, S8
(c) Phosphorous molecule,
P4 (Atomic mass of phosphorous = 31)
(d) Hydrochloric acid,
HCl
(e) Nitric acid, HNO3.
Answer
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