Atoms and Molecules

PAGE 32
Ans : 1
In the given reaction, sodium carbonate reacts with ethanoic acid to produce sodium ethanoate, carbon dioxide, and water.

Mass of sodium carbonate = 5.3 g (Given)
Mass of ethanoic acid = 6 g (Given)
Mass of sodium ethanoate = 8.2 g (Given)
Mass of carbon dioxide = 2.2 g (Given)
Mass of water = 0.9 g (Given)
Now,total mass before the reaction = (5.3 + 6) g
= 11.3 g
And, total mass after the reaction = (8.2 + 2.2 + 0.9) g
= 11.3 g
∴Total mass before the reaction = Total mass after the reaction
Hence, the given observations are in agreement with the law of conservation of mass.

PAGE 33
Ans : 2
It is given that the ratio of hydrogen and oxygen by mass to form water is 1:8.
Then, the mass of oxygen gas required to react completely with 1 g of hydrogen gas is 8 g.
Therefore, the mass of oxygen gas required to react completely with 3 g of hydrogen gas is 8 × 3 g = 24 g.

Ans : 3
The postulate of Dalton’s atomic theory which is a result of the law of conservation of mass is:
Atoms are indivisible particles, which can neither be created nor destroyed in a chemical reaction.

Ans: 4
The postulate of Dalton’s atomic theory which can explain the law of definite proportion is:
The relative number and kind of atoms in a given compound remains constant.

PAGE 35

Ans : 1
Mass unit equal to exactly one-twelfth the mass of one atom of carbon-12 is called one atomic mass unit. It is written as ‘u’.

Ans : 2
The size of an atom is so small that it is not possible to see it with naked eyes. Also, the atom of an element does not exist independently.

PAGE 39
Ans : 1
(i) Sodium oxide →Na₂O
(ii) Aluminium chloride → AlCl₃
(iii) Sodium sulphide → Na₂S
(iv) Magnesium hydroxide → Mg(OH)₂

Ans : 2
(i) Al₂(SO₄)₃→Aluminium sulphate
(ii) CaCl₂→Calcium chloride
(iii) K₂SO₄→Potassium sulphate
(iv) KNO₃→Potassium nitrate
(v) CaCO₃→Calcium carbonate

Ans : 3
The chemical formula of a compound means the symbolic representation of the composition of a compound. From the chemical formula of a compound, we can know the number and kinds of atoms of different elements that constitute the compound.
For example, from the chemical formula CO₂of carbon dioxide, we come to know that one carbon atom and two oxygen atoms are chemically bonded together to form one molecule of the compound, carbon dioxide.

Ans : 4
(i) In an H₂S molecule, three atoms are present; two of hydrogen and one of sulphur.
(ii) In a PO₄³⁻ion, five atoms are present; one of phosphorus and four of oxygen.

PAGE 40
Ans : 1
Molecular mass of H₂= 2 ×Atomic mass of H
= 2 ×1
= 2 u
Molecular mass of O₂= 2 ×Atomic mass of O
= 2 ×16
= 32 u
Molecular mass of Cl₂= 2 ×Atomic mass of Cl
= 2 ×35.5
= 71 u
Molecular mass of CO₂= Atomic mass of C + 2 ×Atomic mass of O
= 12 + 2 ×16
= 44 u
Molecular mass of CH₄= Atomic mass of C + 4 ×Atomic mass of H
= 12 + 4 ×1
= 16 u
Molecular mass of C₂H₆= 2 ×Atomic mass of C + 6 ×Atomic mass of H
= 2 ×12 + 6 ×1
= 30 u
Molecular mass of C₂H₄= 2 ×Atomic mass of C + 4 ×Atomic mass of H
= 2 ×12 + 4 ×1
= 28 u
Molecular mass of NH₃= Atomic mass of N + 3 ×Atomic mass of H
= 14 + 3 ×1
= 17 u
Molecular mass of CH₃OH = Atomic mass of C + 4 ×Atomic mass of H + Atomic mass of O
= 12 + 4 ×1 + 16
= 32 u

Ans : 2

Formula unit mass of ZnO = Atomic mass of Zn + Atomic mass of O
= 65 + 16
= 81 u
Formula unit mass of Na₂O = 2 ×Atomic mass of Na + Atomic mass of O
= 2 ×23 + 16
= 62 u
Formula unit mass of K₂CO₃= 2 ×Atomic mass of K + Atomic mass of C + 3 ×Atomic mass of O
= 2 ×39 + 12 + 3 ×16
= 138 u

PAGE 42
Ans : 1

One mole of carbon atoms weighs 12 g (Given)

i.e., mass of 1 mole of carbon atoms = 12 g

Then, mass of number of carbon atoms = 12 g

Therefore, mass of 1 atom of carbon

 

Ans : 2

Atomic mass of Na = 23 u (Given)

Then, gram atomic mass of Na = 23 g

Now, 23 g of Na contains = number of atoms

Thus, 100 g of Na contains number of atoms

= number of atoms

Again, atomic mass of Fe = 56 u(Given)

Then, gram atomic mass of Fe = 56 g

Now, 56 g of Fe contains = number of atoms

Thus, 100 g of Fe contains number of atoms

number of atoms

Therefore, 100 grams of sodium contain more number of atoms than 100 grams of iron.

 

PAGE 43

Ans : 1

 

Mass of boron = 0.096 g (Given)
Mass of oxygen = 0.144 g (Given)
Mass of sample = 0.24 g (Given)
Thus, percentage of boron by weight in the compound =
= 40%
And, percentage of oxygen by weight in the compound =
= 60%

 

Ans : 2

Carbon + OxygenCarbon dioxide
3 g of carbon reacts with 8 g of oxygen to produce 11 g of carbon dioxide.
If 3 g of carbon is burnt in 50 g of oxygen, then 3 g of carbon will react with 8 g of oxygen. The remaining 42 g of oxygen will be left un-reactive.
In this case also, only 11 g of carbon dioxide will be formed.
The above answer is governed by the law of constant proportions.

PAGE 44

Ans : 3
A polyatomic ion is a group of atoms carrying a charge (positive or negative). For example, ammonium ion, hydroxide ion (OH⁻),carbonate ion, sulphate ion.

Ans : 4
 a) Magnesium chloride →MgCl₂
(b) Calcium oxide →CaO
(c) Copper nitrate →Cu (NO₃)₂
(d) Aluminium chloride →AlCl₃
(e) Calcium carbonate →CaCO₃

Ans : 5

Compound	Chemical formula	Elements present
Quick lime	CaO	Calcium, Oxygen
Hydrogen bromide	HBr	Hydrogen, Bromine
Baking powder	NaHCO3	Sodium, Hydrogen, Carbon, Oxygen
Potassium sulphate	K2SO4	Potassium, Sulphur, Oxygen

Ans : 6
(a) Molar mass of ethyne, C₂H₂ = 2 × 12 + 2 × 1 = 26 g
(b) Molar mass of sulphur molecule, S₈ = 8 × 32 = 256 g
(c) Molar mass of phosphorus molecule, P₄ = 4 × 31 = 124 g
(d) Molar mass of hydrochloric acid, HCl = 1 + 35.5 = 36.5 g
(e) Molar mass of nitric acid, HNO₃ = 1 + 14 + 3 × 16 = 63 g

Ans : 7
(a) The mass of 1 mole of nitrogen atoms is 14 g.
(b) The mass of 4 moles of aluminium atoms is (4 × 27) g = 108 g
(c) The mass of 10 moles of sodium sulphite (Na₂SO₃) is
10 × [2 × 23 + 32 + 3 × 16] g = 10 × 126 g = 1260 g

Ans : 8
(a) 32 g of oxygen gas = 1 mole
Then, 12 g of oxygen gas == 0.375 mole
(b) 18 g of water = 1 mole
Then, 20 g of water == 1.11 moles (approx)
(c) 44 g of carbon dioxide = 1 mole
Then, 22 g of carbon dioxide == 0.5 mole
Ans : 9
(a) Mass of one mole of oxygen atoms = 16 g
Then, mass of 0.2 mole of oxygen atoms = 0.2 × 16g = 3.2 g
(b) Mass of one mole of water molecule = 18 g
Then, mass of 0.5 mole of water molecules = 0.5 × 18 g = 9 g

Ans : 10
1 mole of solid sulphur (S₈) = 8 × 32 g = 256 g
i.e., 256 g of solid sulphur contains = 6.022 × 10²³molecules
Then, 16 g of solid sulphur contains =
= 3.76 × 10²² molecules (approx)

Ans : 11
1 mole of aluminium oxide (Al₂O₃) = 2 × 27 + 3 × 16
= 102 g
i.e., 102 g of Al₂O₃= 6.022 × 10²³molecules of Al₂O₃
Then, 0.051 g of Al₂O₃contains =
= 3.011 × 10²⁰molecules of Al₂O₃
The number of aluminium ions (Al³⁺) present in one molecule of aluminium oxide is 2.
Therefore, the number of aluminium ions (Al³⁺) present in 3.011 × 10²⁰molecules (0.051 g ) of aluminium oxide (Al₂O₃) = 2 × 3.011 × 10²⁰
= 6.022 × 10²⁰