Expansion: 1 Mark
IUPAC stands for International Union of Pure and Applied Chemistry. It is an international scientific organisation which approves names of elements, symbols and units.

Each point: 1 Mark
1. Atomicity is the total number of atoms present in one molecule of an element or a substance.
2. a. The chemical compound ozone is composed of 3 oxygen atoms. Therefore atomicity is three.
    b. The chemical formula of ammonia is ${NH}_3$ in which there are 3 Hydrogen atoms and 1 Nitrogen atom. Therefore the atomicity is four.

Each point : 1 Mark
1. Oxygen is a mixture of O-16 , O-17 and O-18 isotopes, which gives the average mass if oxygen is taken as the standard.
2. C-12 is the most abundant isotope of carbon which constitutes 98.9% of available isotopes of carbon.
So it was better to choose Carbon-12 isotope as the standard.

Each point: 1.5 Marks
1. Atomic mass of Na = 23 u (Given)

Then, gram atomic mass of Na = 23 g

Now, 23 g of Na contains = 6.022 x 10²³ g number of atoms

Thus, 100 g of Na contains =   $6.022\times \frac{{10}^{23}}{23}\times 100$ number of atoms

= 2.6182 x 10²⁴ number of atoms.

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

Then, gram atomic mass of Fe = 56 g

Now, 56 g of Fe contains = 6.022 x 10²³ g number of atoms

Thus, 100 g of Fe contains = $6.022\times \frac{{10}^{23}}{56}\times 100$ number of atoms

= 1.0753 x 10²⁴ number of atoms.

Each point: 1 Mark
1. Mass of one mole of elemental oxygen atom = 16 g
Then, mass of 0.2 mole of oxygen atoms = 0.2 x 16g = 3.2 g

2. Mass of one mole of water molecule = (2 x 1 g)+ 16 g =18 g
Then, mass of 0.5 mole of water molecules = 0.5 x 18 g = 9 g

Statement : 1 Mark each
The two laws of chemical combination are
1. Law of conservation of mass: 
The law of conservation of mass states that mass in an isolated system is neither created nor destroyed by chemical reactions or physical transformations. According to the law of conservation of mass, the mass of the products in a chemical reaction must equal the mass of the reactants.
2. Law of definite proportions:
The law of definite proportions, also called Proust's law or the law of definite composition, or law of constant composition states that a given chemical compound always contains its component elements in fixed ratio (by weight) and does not depend on its source and method of preparation.

Each point: 1 Mark
1. Charge on ammonium(${N{H}_4}^{+}$) is +1

2. Charge on sulphate(${S{O}_4}^{2-}$) is -2

3. By cross over charges to balance charges

$(N{H}_4{)}_{2}S{O}_4$.

Concept : 1 Mark
Process : 1 Mark 
Result : 1 Mark
Carbon and chlorine will combine in such a manner that their valencies should get satisfied. 
Valency of Carbon = 4
Valency of Chlorine = 1
To satisfy the valency of Carbon, 4 chlorine atoms each having a valency of 1 would be required. Hence, the formula is $CC{l}_4$

Each step: 1 Mark
1. Valency of  aluminium = 3

2. Valency of chlorine =1

3. By crossing over the valencies we have $Al{Cl}_3$.

Each point: 1 Mark
1. The chemical formula of a compound is a symbolic representation of its composition.

2. Mole of aluminium oxide (Al₂O₃) = 2 x 27 + 3 x 16

= 102 g

3. i.e., 102 g of Al₂O₃= 6.022 x 10²³ molecules of Al₂O₃

4. Then, 0.051 g of Al₂O₃ contains = $6.022\times \frac{{10}^{23}}{102}\times 0.051$ molecules

= 3.011 x 10²⁰ molecules of Al₂O₃

5. 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 x 10²⁰ molecules (0.051 g ) of aluminium oxide (Al₂O₃) = 2 x 3.011 x 10²⁰

= 6.022 x 10²⁰