10.03.04 Moles and mass in reactions
Syllabus
The masses of reactants and products can be calculated from balanced symbol equations.
Chemical equations can be interpreted in terms of moles. For example:
Mg + 2HCI --> MgCI2 + H2
shows that one mole of magnesium reacts with two moles of Hydrochloric acid to produce one mole of Magnesium Chloride and one mole of Hydrogen gas.
Students should be able to:
calculate the masses of substances shown in a balanced symbol equation
calculate the masses of reactants and products from the balanced symbol equation and the mass of a given reactant or product.
What does this mean?
We know from the equation
C(s) + O2(g) --> CO2 (g)
that 1 mole of Carbon reacts with 1 mole of Oxygen to make 1 mole of CO2.
And we also know that the Molar Mass (mass of one mole) of Carbon = Ar (Carbon) - the Mass Number from the Periodic Table = 12
And that the Molar Mass (mass of one mole) of O2
= 2 x Ar (O) = 2 x 16 = 32.
And that the Molar Mass (mass of one mole) of CO2
= 1 x Ar (C) + 2 x Ar (O) = 12 + (2 x 16) = 44.
So, we can say 12g of Carbon will need exactly 32g of Oxygen to burn and will make 44g of Carbon Dioxide
But how will we know how much Oxygen we need if we want to burn more (or less) than 12 g of Carbon?
We must work out how many moles of Carbon we have to burn using the equation:
Moles = Mass ÷ Molar Mass
If we wish to burn 18 g of Carbon:
Moles of Carbon = Mass Carbon ÷ Molar Mass (C)
= 18 ÷ 12 = 1.5 moles of Carbon
1 mole of Carbon needs 1 mole of O2 so 1.5 moles of Carbon will need 1.5 moles of O2
Moles of O2= Mass O2 ÷ Molar Mass (O2)
Mass of O2 needed = Moles of O2 needed X Molar Mass (O2)
= 1.5 x 32 = 48 g
Burning 1.5 moles of Carbon will also make 1.5 moles of CO2. How much is this?
Moles of CO2 = Mass CO2 ÷ Molar Mass (CO2)
Mass of CO2 = Moles of CO2 X Molar Mass (CO2) Mr (CO2) = Ar (C) + 2Ar (O) = 12 + 32 = 44 g/mol
= 1.5 x 44 = 66 g
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