Formula Stoichiometry

Percentage Composition

If you know the formula of a compound, you can find out how much of each of the elements is present in a given quantity of the compound. For example, in Al₂O₃ (bauxite), you can calculate how much aluminium can be obtained.

Molar mass of Al₂O₃ = (2 × 27.0) g + (3 × 16.0) g = 102.0 g

Since 1 mol of Al₂O₃ contains 2 mol of Al atoms, the mass of Al is 2 × 27.0 g = 54.0 g Al

Percentage of Aluminium = 54.0/102.0 = 59.9%

Similarly, percentage of oxygen = 48.0/102.0 = 47.1%

Formula Stoichiometry

Example: Water

Water consists of 11.11% hydrogen and 88.89% oxygen by mass. From this data, you can determine empirical formula of water.

If you assume that you have a 100.00 g sample of water, then by the percentage composition, 100.0 g of water contains 11.11 g of hydrogen atoms and 88.89 g of oxygen atoms. 1 mol of hydrogn atoms has a mass of 1.0 g, and 1 mol of oxygen atoms has a mass of 16.0 g.

11.11 g H contains 11.11 mol H atoms. 88.89 g O contains 5.55 mol O atoms.

The ratio of moles of hydrogen atoms to moles of oxygen atoms is 11.11 : 5.55.

Thus, ratio hydrogen and oxygen atoms in water is 2 : 1 and empirical formula of water is H₂O.