3.2 Moles and chemical equations
The concept of the mole gives an alternative interpretation of the chemical equation.
Look again at the following equation for the combustion of methane:
CH4 (g) + 2O2 (g) = CO2 (g) + 2H2O(l)
Explain in a single sentence what this equation means to you.
This equation represents an overview of a chemical change in which molecules of methane and oxygen are converted to molecules of carbon dioxide and water.
This is the molecular interpretation of the equation, but you could also view it somewhat differently.
The equation can also represent a process in which one mole of methane molecules and two moles of oxygen molecules react together to form one mole of carbon dioxide and two moles of water.
This is a molar interpretation of the equation.
Using the mole, it is possible to calculate quantities of reactants and products from a chemical equation.
Methane is the major component of natural gas. The average house in the UK burns about 1000 kg of methane each year. How much carbon dioxide enters the atmosphere from this source?
The relative molecular mass of methane is 16.0 and that of carbon dioxide is 44.0.
So the mass of one mole of methane molecules is 16.0 g and the mass of one mole of carbon dioxide molecules is 44.0 g.
One mole CH4 is converted to one mole CO2.
So 16.0 g CH4 are converted to 44.0 g CO2.
1.00 g CH4 is converted to 44.0/16.0 g CO2
1.00 kg CH4 is converted to 44.0/16.0 kg CO2
1000 kg CH4 is converted to 1000 × 44.0/16.0 kg CO2= 2.75 × 103 kg
So on average, each house adds 2750 kg of carbon dioxide to the atmosphere each year.
So have seen that using the concept of the mole in conjunction with balanced chemical equations allows the masses of reactants and products to be calculated. However much of the work carried out by chemists take place in solution, and as such there is a need to be able to quantify how much of a particular substance is dissolved in a liquid – to this end, being able to work in moles is crucial. This is the focus of the next section.