(a) Molecular mass:

The molecular mass of a substance is the sum of atomic masses of all the atoms in a molecule of the substance. It is used for those substances whose constituent particles are molecules.

For example: The molecular mass of CO₂ = 1× atomic mass of carbon + 2× atomic mass of oxygen

⇒ 12 + 2 × 16

⇒ 12 + 32 = 44u

(b) Chemical formula:

The chemical formula of a compound is a symbolic representation of its composition. The chemical formula for different compounds can be written easily. For writing a chemical formula, we need to learn the symbols and combining the capacity of elements.

For example Chemical formula of carbon tetrachloride:

Valency of carbon is 4

Valency of chlorine is 1

By applying the criss-cross method:

Formula: CCl₄

Thus, the formula for carbon tetrachloride is CCl₄.

(c) One mole of any species (atoms, ions or molecules) having that quantity in number having a mass equal to its atomic or molecular mass in grams.