C2aL12 Percentage Composition

Key Words

Atom - smallest particle into which an element can be chopped.

Formula - short-hand showing the elements and the number of atoms there are in the compound.

Percentage - fraction multiplied by 100.

Ratio - proportion of one element compared with another.

Relative Formula Mass - mass of a compound worked out from the atomic masses of its elements.

Test Yourself

Homework
Subject Page
Home

Grade E

Chemists sometimes want to know how much of a particular element is in a compound.  Potassium permanganate is a purple compound that has the formula KMnO4.  A chemist might want to know what percentage of potassium permanganate is potassium. 

It is NOT 1/3, nor is it 1/6.

Let's see how to do it.

Firstly we need to work out the relative formula mass:

  • 1 atom of potassium, K, relative atomic mass 39;
  • 1 atom of manganese, Mn, relative atomic mass 55;
  • 4 atoms of oxygen, O, relative mass 16 to give overall mass of 64.

Add these together:

Relative formula mass = 39 + 55 + 64 = 158

To get the percentage, we need to use the numerical equation:

The relative atomic mass of potassium is 39, and there is 1 atom, so we put in the numbers:

       % mass K = 39 × 1  × 100 =  24.7 %

158

If we did the same for manganese, we would find the percentage was 35.5 %, while for oxygen it would be 40.5 %.  If you add these numbers up, you will find that the total is 100.7 %.  This is because of rounding errors.

The ratio of potassium : manganese : oxygen is approximately 25 : 35 : 40.

 

Grade C

We can apply a similar method for any compound we want.  Consider magnesium hydroxide, Mg (OH)2.  What is the proportion of oxygen?

Relative formula mass = 23 + 2 × (16 + 1) = 23 + 34 = 57.

There are two oxygen atoms in the formula, so:

% composition of oxygen = 16 × 2 × 100 = 56.1 %

                    57

Grade A

Percentage compositions are often given in fertilisers, enabling farmers to work out the optimum amount of fertiliser to apply to their fields.  They don't want to put too much on because:

  • It's a waste of money;
  • Too much fertiliser can end up in run-off, causing pollution of waterways.
  • Too much fertiliser can scorch plants.