Definition: 2 Marks
Numerical: 3 Marks
One mole is defined as the amount of substance containing as many elementary entities (atoms, molecules, ions, electrons, radicals, etc.) as there are atoms in 12 grams of carbon-12. The number of particles that is atoms, ions and molecules  present in one mole of a substance is fixed with a value of $6.02\times {10}^{23}$ molecules.
Mole of X element is ;
$nx=(9.03\times {10}^{23}\times (6.02\times {10}^{23})=0.15mol$
Atomic mass of X;
$A_x=\frac{8.4}{0.15}=56g/mol$
Molar mass of $X_2{Y}_3$ compound;
$M_{X_2{Y}_3}=\frac{16}{0.1}=160g/mol$
We find atomic mass of Y by;
2.X + 3Y =160
2.(56) + 3(Y)=160
Y=16g/mol is atomic mass of Y.

Each point: 1 Mark
(a) Ethyne, C₂H₂
Molar mass of ethyne, C₂H₂ = 2 x 12 + 2 x 1 = 26 g

(b) Sulphur molecule, S₈
Molar mass of sulphur molecule, S₈ = 8 x 32 = 256 g

(c) Phosphorus molecule, P₄ (atomic mass of phosphorus = 31)
Molar mass of phosphorus molecule, P₄ = 4 x 31 = 124 g

(d) Hydrochloric acid, HCl
Molar mass of hydrochloric acid, HCl = 1 + 35.5 = 36.5 g

(e) Nitric acid, HNO₃
Molar mass of nitric acid, HNO₃ = 1 + 14 + 3 x 16 = 63 g

Explanation - 1 Mark
Problem - 2 Marks
1. Elements combine to form chemical compounds and attain stability. For example, metals often react with nonmetals to form ionic compounds. These compounds are composed of positive and negative ions formed by adding or subtracting electrons from neutral atoms and molecules. Thus these compounds are stable in nature.

2. 1 mole of solid sulphur ($S_8$ ) = 8 x 32 g = 256 g

i.e., 256 g of solid sulphur contains = 6.022 x 10²³ molecules

Then, 16 g of solid sulphur contains = $6.022\times \frac{{10}^{23}}{256}\times 16$

= 3.76375 x 10²² molecules

Each point: 1 Mark
1. The atomic mass is the mass of an atom of a chemical element expressed in atomic mass units. It is equivalent to number of protons and neutrons in the atom.
2. 1 unified atomic mass unit is defined as $\frac{1}{12}$ of the mass of a single carbon-12 atom.

Definition: 1 Mark
Example: 1 Mark
A molecule can be defined as the smallest particle of an element or a compound that is capable of an independent existence and shows all the properties of that substance.
For example oxygen molecule is written as (O₂)