Calculate the molecular masses of H\( _2 \) , O\( _2 \) , Cl\( _2 \), CO\( _2 \), CH\( _4 \), C\( _2 \)H\( _6 \), C\( _2 \)H\( _4 \), NH\( _3 \), CH\( _3 \)OH.

We know that molecular mass of a substance is the sum of the atomic masses of all the atoms in the molecule of the substance.

The molecular mass of H\( _2 \)  = atomic mass of H + atomic mass of H = 2 × atoms atomic mass of H = 2 × 1 = 2u

The molecular mass of O\( _2 \)  = 2 × atoms atomic mass of O = 2 × 16 = 32u

The molecular mass of Cl\( _2 \)  = 2 × atoms atomic mass of Cl = 2 × 35.5 = 71u

The molecular mass of CO\( _2 \)   = atomic mass of C + 2 × atomic mass of O = 12 + ( 2 × 16) = 44u

The molecular mass of CH\( _4 \)  = atomic mass of C + 4 × atomic mass of H = 12 + ( 4 × 1) = 16u

The molecular mass of C\( _2 \)H\( _6 \)  = 2 × atomic mass of C + 6 × atomic mass of H = (2 × 12) + (6 × 1) = 24 + 6 = 30u

The molecular mass of C\( _2 \)H\( _4 \)  = 2 × atomic mass of C + 4 × atomic mass of H = (2 × 12) + (4 × 1) = 24 + 4 = 28u

The molecular mass of NH\( _3 \)  = atomic mass of N + 3 × atomic mass of H = 14 + (3 × 1) = 17u

The molecular mass of CH\( _3 \)OH   = atomic mass of C + 3 × atomic mass of H + atomic mass of O + atomic mass of H = 12 + (3 × 1) + 16 + 1 = 12 + 3 + 17 = 32u