GCSE Chemistry Unit 1 Calculations

There are a number of types of calculation you need to know, these are fundamental and will be examined in Unit 1 but could still be examined in Unit 2:


1.) Be able to calculate the relative molecular (formula) mass (Mr) of a compound from its
     formula.
    
Every element in the periodic table is represented by a symbol and numbers:

The top number is the mass number (or nucleon number), it gives the total mass of the atom. 
     
So oxygen has a mass of 16 and hydrogen has a mass of 1.

The relative molecular (formula) mass is the sum of masses of all atoms within the molecule or compound. 

So for water, H₂O the relative molecular mass is:
     

     Mr = 2 x H + 1 x O = (2 x 1) + (1 x 16) = 18

2.) Calculate the percentage composition of simple compounds.

3.) Collect experimental data, and use given data, in order to calculate the formula of 

     a binary compound e.g. magnesium oxide. (Higher Tier)

You could be asked to find the empirical formula of a compound given either the mass or percentage of the elements in that formula.  The following clip explains how the calculations are carried out (The first 2 minutes and 40 seconds are the most important):

4.)  Be able to calculate the percentage yield of a reaction.

The clip below talks through how to calculate percentage yield (the first 2 minutes and 50 seconds are the most important):