Reading masses from an equation
A balanced equation does not merely count atoms — masses can be read from it too. Below is the reaction between haematite (Fe2O3) and carbon monoxide.
Fe2O3 + 3CO → 2Fe + 3CO2
The molar masses first
Given the relative atomic masses Fe = 56, O = 16 and C = 12:
Fe2O3 = (2 × 56) + (3 × 16) = 160 g mol−1
CO = 12 + 16 = 28 g mol−1
CO2 = 12 + (2 × 16) = 44 g mol−1
What the equation tells us
(a) The carbon monoxide needed
Three moles of CO are needed to react with one mole of Fe2O3.
So 28 × 3 g = 84 g of CO is needed to react with 160 g of Fe2O3.
(b) The iron formed
Two moles of iron are formed in the above reaction.
So 56 × 2 g = 112 g of iron is formed.
(c) The carbon dioxide formed
Three moles of carbon dioxide are formed in the above reaction.
So 44 × 3 g = 132 g of carbon dioxide is formed.
Does the mass balance?
Mass in = 160 + 84 = 244 g. Mass out = 112 + 132 = 244 g. By the law of conservation of mass the two must be equal — and they are.
Coefficients are moles
The coefficients in a balanced equation give the number of moles taking part. Multiply each by its molar mass and you have the mass ratio.