Question

(a) What is meant by the 'molar mass' of a substance ? State the unit in which molar mass is usually expressed.
(b) Calculate the molar masses of the following substances. Write the results with proper units.

(i) Ozone molecule, O₃
(ii) Ethanoic acid, CH₃COOH

Answer 1

(a) Molar mass of a substance (molecular compound) is the mass in grams, numerically equivalent to the sum of atomic masses of individual atoms in the molecular formula of that substance.
Molar mass of a substance is usually expressed as grams/mole.

(b)

(i) Molar mass of ozone (O₃)
Molar mass of ozone = 16 + 16 +16 = 48 g/mol

(ii) Molar mass of acetic acid (CH₃COOH)
Molar mass of acetic acid = (2 × 12) + (4 × 1) + (2 × 16) = 60 g/mol