3.2.3 Using moles to balance equations
Measured masses can reveal the balancing numbers
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.
- Higher tier only: balancing from masses is assessed on the higher tier.
- If you know the mass of each substance that reacts, you can work out the balancing numbers of the equation.
- The method is to turn each mass into moles and then find the simplest ratio of those moles.
Key Idea
The simplest whole-number ratio of the moles is exactly the set of balancing numbers in the equation.
First change every mass into moles
Definition
Relative formula mass
The sum of the relative atomic masses of all the atoms shown in a chemical formula.
- Work out the moles of each reactant and product using n=mMrn = \dfrac{m}{M_r}n=Mrm.
- Keep every value in moles, since only moles can be compared as a ratio.
Example
- 48 g48\ \text{g}48 g of magnesium (Ar=24A_r = 24Ar=24) is 4824=2 mol\dfrac{48}{24} = 2\ \text{mol}2448=2 mol.
- 32 g32\ \text{g}32 g of oxygen (Mr=32M_r = 32Mr=32) is 3232=1 mol\dfrac{32}{32} = 1\ \text{mol}3232=1 mol.
Then reduce the moles to the simplest whole-number ratio
Definition
Mole ratio
The ratio of reacting amounts shown by the balancing numbers in a chemical equation.
- Divide every mole value by the smallest one to get the simplest ratio.
- Round the ratio to whole numbers, which then become the balancing numbers.
Exam technique
- Divide by the smallest number of moles first, as it makes the ratio easiest to read.
- A ratio like 1:0.51:0.51:0.5 is not finished, so double both numbers to reach whole numbers such as 2:12:12:1.
Example
- 48 g48\ \text{g}48 g of magnesium reacts with 32 g32\ \text{g}32 g of oxygen to make 80 g80\ \text{g}80 g of magnesium oxide.
- The moles are 222 for magnesium, 111 for oxygen and 8040=2\dfrac{80}{40} = 24080=2 for magnesium oxide.
- The ratio 2:1:22:1:22:1:2 gives the balanced equation 2Mg+O2→2MgO2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO}2Mg+O2→2MgO.
Self review
- What two steps turn reacting masses into balancing numbers?
- Why must the masses be changed into moles before comparing them?
- How do you deal with a mole ratio that comes out as 1:0.51:0.51:0.5?
- 2 mol2\ \text{mol}2 mol of hydrogen reacts with 1 mol1\ \text{mol}1 mol of oxygen; what is the simplest ratio?
