Analysing Substances - Quantitative

A mole represents 6.03 x 10²³ particles of a substance, but it doesn't matter if you can't remember that!. The number is so large because atoms and molecules are so tiny.

The main calculations in quantitative analysis depend on your ability to work out the relative formula mass of any given substance. Most of the time you will be given the correct formula but the highest level candidates will show that they can work out the correct formula for a compound from any data given.

The mole simplifies chemical calculations by telling you the proportions of each substance that are involved in a chemical reaction. Look at the following chemical equation:

2H₂ + O₂ → 2H₂O

The numbers in front of each of the formulae indicate the number of moles required. No number in front indicates 1 mole. So in the above reaction, 2 moles of hydrogen molecules react with 1 mole of oxygen molecules to form 2 moles of water molecules - a ratio of 2:1:2.

To work out quantities you only need to know how many moles of ONE of the substances you have to work out the quantities of the rest. If 4 moles of water were produced in the reaction above, you would need 4 moles of hydrogen molecules and 2 of oxygen. If only 0.5 moles of water were produced, you would need 0.5 moles of hydrogen molecules and 0.25 moles of oxygen.

If you need to work out the number of moles for yourself, you use the equation moles = mass/relative formula mass. You will need to rearrange this as required by the questions that you have to answer.

Have a go at this quiz on analysing substances and see how much you understand about quantitative analysis.