Calculating Moles

Moles and Elements

We can compare the masses of all the other atoms with the mass of carbon atoms and this is the basis of the relative atomic mass scale. Chemists have found by experiment that relative atomic mass of an element in grams always contains 6 × 10²³ or one mole of its atoms.

For example, the relative atomic mass (A_r) of iron (Fe) is 56. One mole of iron is 56 g. Therefore, 56 g of iron will contain 6 × 10²³ atoms.

In this way, the A_r for aluminium (Al) is 27. It is found that 27 g of aluminium contains 6 × 10²³ atoms in it. Thus 27 g of aluminium is one mole of aluminium atoms.

The mass of a substance present in any number of moles can be calculated using the following relationship:

Example 1

Calculate the mass of a) 2 moles and b) 0.25 mole of iron. (A_r For Fe is 56)

a) The mass of 2 moles of iron

= number of moles × relative atomic mass (A_r)

= 2 × 56 = 112 g

b) The mass of 0.25 mole of iron

= number of moles × relative atomic mass (A_r)

= 0.25 × 56 = 14 g.

If we know the mass of the element then it is possible to calculate the number of moles of that element using this relationship:

Related: Chemical Calculations: Relative Atomic Mass and Reacting Mass

Example 2

We can calculate the number of moles of aluminium present in a) 108 g and b) 13.5 g of the element. (A_r For Al is 27)

a) number of moles of (108 g) aluminium is

= 108 ÷ 27 = 4 moles

b) number of moles of 13.5 g aluminium is

= 13.5 ÷ 27 = 0.5 mole

Source- Chembridge IGCSE Chemistry