Combustion Calculations

 Finding empirical formulae from combustion data

If you burn a compound containing carbon and hydrogen in an excess of air or oxygen, carbon dioxide and water are formed. If you can trap the water and carbon dioxide separately, you can find out what mass of each is formed. From that, you can work out the empirical formula.

How you do this varies slightly depending on whether the compound contains anything else as well as the carbon and hydrogen. We'll look at the two cases separately.

If the compound only contains carbon and hydrogen

A compound which only contains carbon and hydrogen is called a hydrocarbon. Look for this word in the question. You can only use this shorter method if you know that there is nothing else present.

Here's an example:

When 0.78 g of a hydrocarbon was burned in excess air, 2.64 g of carbon dioxide and 0.54 g of water were formed. Find the empirical formula of the hydrocarbon.

The important things to notice is that every mole of CO₂ contains 1 mole of carbon atoms. Every mole of H₂O contains 2 moles of hydrogen atoms.

1 mole of CO₂ weighs 44 g.

No of moles of CO₂ = 2.64/44 = 0.06

Therefore, no of moles of carbon atoms, C = 0.06

1 mole of H₂O weighs 18 g.

No of moles of H₂O = 0.54/18 = 0.03

Each mole of H₂O contains 2 moles of hydrogen atoms.

Therefore, no of moles of hydrogen atoms, H = 0.06

The ratio of the number…