Percentage composition- Percentage composition gives the mass of each element expressed as the percentage of the total mass. It means it gives the number of grams of the element present in 100 gm of the compound.

Mass percentage of an element = mass of that element in 1 mole compound x 100 / Molar mass of the compound

Ex 1) Calculate the mass percentage composition of water(H2O).

Solution –

Molar mass 0f water = 2×1 + 16 = 18

Mass % of an element = mass of that element in 1 mole compound x 100 / Molar mass of the compound

Mass % of hydrogen = 2 x 100 / 18

Mass % of hydrogen =11.11 %

Mass % of oxygen = 16 x 100 / 18

Mass % of oxygen =88.89%

Ex 2) Calculate the mass percentage composition of sucrose (C12H22O11).

Solution –

Molar mass 0f sucrose = 12 x 12 + 22×1 + 11 x 16= 342

Mass % of an element = mass of that element in 1 mole compound x 100 / Molar mass of the compound

Mass % of Carbon = 144 x 100 / 342

Mass % of carbon = 42.11 %

Mass % of hydrogen = 22 x 100 / 342

Mass % of hydrogen = 6.43 %

Mass % of oxygen = 176 x 100 / 342

Mass % of oxygen =51.46%

Ex 3) Calculate the mass percentage composition of urea (NH2CONH2).

Solution –

Molar mass 0f urea(NH2CONH2 or N2H4CO)= 14×2 + 4×1 + 12 + 16= 60

Mass % of an element = mass of that element in 1 mole compound x 100 / Molar mass of the compound

Mass % of nitrogen = 28 x 100 / 60

Mass % of nitrogen =46.67  %

Mass % of Carbon = 12 x 100 / 60

Mass % of carbon = 20 %

Mass % of hydrogen = 4 x 100 / 60

Mass % of hydrogen =6.67 %

Mass % of oxygen = 16 x 100 / 60

Mass % of oxygen = 26.67 %

Ex 4) Calculate the mass percentage composition of ethyl alcohol or ethanol (C2H5OH).

Solution –

Molar mass 0f ethyl alcohol = 2 x 12 + 6×1 + 1 x 16= 46

Mass % of an element = mass of that element in 1 mole compound x 100 / Molar mass of the compound

Mass % of Carbon =24 x 100 / 46

Mass % of carbon = 52.17 %

Mass % of hydrogen = 6 x 100 / 46

Mass % of hydrogen = 13.04 %

Mass % of oxygen = 16 x 100 / 46

Mass % of oxygen = 34.78 %

Ex 5) Calculate the mass percentage composition of Copper pyrite (CuFeS2)

Solution –

Molar mass 0f Copper pyrite =63.5 + 56 + 2 x 32 = 183.5

Mass % of an element = mass of that element in 1 mole compound x 100 / Molar mass of the compound

Mass % of Copper = 63.5 x 100 / 183.5

Mass % of Copper = 34.60 %

Mass % of Iron = 56 x 100 / 183.5

Mass % of iron = 30.52 %

Mass % of Sulphur = 64 x 100 / 183.5

Mass % of sulphur = 34.88 %