3.2.2 Amounts of substances in equations
The balancing numbers give the ratio of moles that react
Definition
Mole
The amount of a substance, measured in mol, where one mole has a mass in grams numerically equal to its relative formula mass.
Definition
Mole ratio
The ratio of reacting amounts shown by the balancing numbers in a chemical equation.
- The large numbers in front of the formulae give the mole ratio in which the substances react.
- In 2H2+O2→2H2O2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O}2H2+O2→2H2O, two moles of hydrogen react with one mole of oxygen to make two moles of water.
Key Idea
A balanced equation is a recipe in moles, so once you know the moles of one substance you can find the moles of any other.
Three steps take you from one mass to another
Definition
Relative formula mass
The sum of the relative atomic masses of all the atoms shown in a chemical formula.
- Change the mass you are given into moles using n=mMrn = \dfrac{m}{M_r}n=Mrm.
- Use the mole ratio from the balanced equation to find the moles of the substance you want.
- Change those moles back into a mass using m=n×Mrm = n \times M_rm=n×Mr.
Exam technique
- Always convert to moles before using the ratio, as you cannot apply the balancing numbers directly to masses.
- Set out each stage on its own line, because staged calculations earn marks even if the final answer slips.
Example
- Find the mass of magnesium oxide made when 6 g6\ \text{g}6 g of magnesium burns in 2Mg+O2→2MgO2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO}2Mg+O2→2MgO.
- Moles of magnesium =624=0.25 mol= \dfrac{6}{24} = 0.25\ \text{mol}=246=0.25 mol.
- The ratio of magnesium to magnesium oxide is 2:22:22:2, so moles of magnesium oxide =0.25 mol= 0.25\ \text{mol}=0.25 mol.
- Mass of magnesium oxide =0.25×40=10 g= 0.25 \times 40 = 10\ \text{g}=0.25×40=10 g.
Self review
- What does the mole ratio in a balanced equation tell you?
- Why must you change a mass into moles before using the ratio?
- In 2Mg+O2→2MgO2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO}2Mg+O2→2MgO, how many moles of oxygen react with 0.25 mol0.25\ \text{mol}0.25 mol of magnesium?
- Find the mass of water made from 4 g4\ \text{g}4 g of hydrogen in 2H2+O2→2H2O2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O}2H2+O2→2H2O.
