Calculate the molecular masses of H2, O2, Cl2, C02, CH4, C2H6, C2H4, NH3, CH30H (Atomic masses: H = 1; 0 = 16; Cl = 35.5; C = 12; N = 14)

The molecular mass of a substance is the sum of the atomic masses of all the atoms in a molecule of the substance. It is therefore the relative mass of a molecule expressed in atomic mass units (u).

(i) Atomic mass of Hydrogen = 1 u.

Molecular mass of H₂ = Mass of 2H atoms = 2 x 1 = 2 u

(ii) Atomic mass of Oxygen = 16 u

Molecular mass of O₂ = Mass of 2 'O' atoms = 2 X 16 = 32 u

(iii) Atomic mass of Cl = 35.5 u

Molecular mass of Cl₂ = Mass of 2Cl atoms = 2 X 35.5 = 71 u

(iv) In this molecule, there is one carbon atom and two oxygen atoms.

Atomic mass of Carbon = 12 u

Atomic mass of Oxygen = 16 u

Molecular mass of C0₂ = Mass of C atom + Mass of 2 '0' atoms

= 12 + 2 X 16 = 12 + 32 = 44 u

(v) Methane molecule has one carbon atom and four hydrogen atoms.

Atomic mass of Carbon = 12 u, Atomic mass of Hydrogen = 1 u

Molecular mass of CH₄ = Mass of C atom + Mass of 4H atoms

= 12 + 4 X 1 = 12 + 4 = 16 u

(vi) C₂H₆ molecule has two carbon atoms and six hydrogen atoms.

Atomic mass of Carbon = 12 u, Atomic mass of Hydrogen = 1 u

Molecular mass of C₂H₆ =Mass of 2C atoms + Mass of 6H atoms

= 2 X 12 + 6 X 1 = 24 + 6 = 30 u

(vii) C₂H₄ molecule has two carbon atoms and four hydrogen atoms.

C₂H₆ molecule has two carbon atoms and four hydrogen atoms.

Molecular mass of C₂H₄ = Mass of 2C atoms + Mass of 4 H atoms

= 2 X 12 + 4 X 1 = 24 + 4 = 28 u

(viii) NH₃ molecule has one nitrogen atom and three hydrogen atoms.

Atomic mass of Nitrogen = 14 , Atomic mass of hydrogen = 1 u

Molecular mass of NH₃ = Mass of N atom + Mass of 3 H atoms

= 14 + 3 X 1 = 14 + 3 = 17 u

(ix) CH₃0H molecule has one carbon atom, four hydrogen atoms and one oxygen atom.

Atomic mass of carbon =n 12 u, Atomic mass of hydrogen = 1 u and Atomic mass of oxygen = 12 u.

Molecular mass of CH₃0H = Mass of C + Mass of 4 H + Mass of 0

= 12 + 4 X 1 + 16 = 12 + 4 + 16 = 32 u