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Empirical and molecular formulae from experiments
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Chemists find formulae by measuring masses. The empirical formula is the simplest ratio; the molecular formula is the actual number of atoms.
Empirical and molecular formulae
Empirical formula: the simplest whole-number ratio of atoms of each element.
Molecular formula: the actual number of atoms of each element in a molecule.
Ethane is C2H6; its empirical formula is CH3.
To get the molecular formula: divide M_r by the empirical formula mass, then multiply the empirical formula by that number.
Molecular formula: the actual number of atoms of each element in a molecule.
Ethane is C2H6; its empirical formula is CH3.
To get the molecular formula: divide M_r by the empirical formula mass, then multiply the empirical formula by that number.
Finding a metal oxide formula
Combustion: heat a weighed piece of magnesium in a crucible, lifting the lid now and then to let air in. Weigh the oxide. Mass of oxygen = mass of oxide − mass of magnesium.
Reduction: heat copper(II) oxide in a stream of a reducing gas (such as hydrogen or methane). Mass of oxygen = mass lost.
Then: moles of each element = mass ÷ A_r, and divide by the smallest.
Reduction: heat copper(II) oxide in a stream of a reducing gas (such as hydrogen or methane). Mass of oxygen = mass lost.
Then: moles of each element = mass ÷ A_r, and divide by the smallest.
Water of crystallisation
Some salts contain water in their crystals, such as hydrated copper(II) sulfate, CuSO4·5H2O.
Heat a weighed sample to constant mass to drive off the water. Mass of water = mass lost.
Find moles of anhydrous salt and moles of water, then the ratio.
Heat a weighed sample to constant mass to drive off the water. Mass of water = mass lost.
Find moles of anhydrous salt and moles of water, then the ratio.
0.24 g of magnesium forms 0.40 g of magnesium oxide. Find the empirical formula. (A_r: Mg = 24, O = 16)
- Mass of O = 0.40 − 0.24 = 0.16 g
- Mg: 0.24 ÷ 24 = 0.01 mol; O: 0.16 ÷ 16 = 0.01 mol
- Ratio 1 : 1
Answer: MgO
Empirical formula is the simplest ratio; molecular formula is the actual number. Divide masses by A_r and by the smallest. Find metal oxide formulae by combustion (magnesium) or reduction (copper(II) oxide). Heat hydrated salts to constant mass to find the water of crystallisation.
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