Quantitative chemistry

You should be able to calculate the masses of reactants and products from balanced equations, and the percentage composition by mass of an element in a compound.

Higher tier students should also be able to calculate the percentage yield of a reaction, and the empirical formula of a compound from information about reacting masses.

Percentage composition

Percentage composition is just a way to describe what proportions of the different elements there are in a compound.

If you have the formula of a compound, you should be able to work out thepercentage by mass of an element in it.

Example

The formula for sodium hydroxide is NaOH. It contains three different elements:Na, O and H. But the percentage by mass of each element is not simply 33.3 per cent, because each element has a different relative atomic mass. You need to use the A_r values to work out the percentages. Here is how to do it:

Question

What is the percentage by mass of oxygen (O) in sodium hydroxide (NaOH)?

• First, work out the relative formula mass of the compound, using the A_r values for each element. In the case of sodium hydroxide, these are Na = 23, O = 16, H = 1. (You will be given these numbers in the exam.)
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• Next, divide the A_r of oxygen by the M_r of NaOH, and multiply by 100 to get a percentage.

Remember, if there is more than one atom of the element in the compound, you need to multiply your answer by the number of atoms. If your answer is more than 100 per cent, you have gone wrong!