Using the mole and balanced symbol equations to calculate reacting masses, the conservation of mass, and finding empirical formulae from mass or percentage data.

What this dot point is asking

CCEA wants you to use the conservation of mass and a balanced equation to calculate the mass of a reactant or product, working through moles, and to find an empirical formula from masses or percentage composition.

Conservation of mass

Apparent mass changes usually mean a gas was lost or gained: when a metal burns the product gains mass by combining with oxygen, while a fizzing reaction that releases gas appears to lose mass.

The reacting-mass method

The ratio in step 2 comes straight from the big numbers in the balanced equation. If the equation says $2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO}$, then 2 moles of magnesium make 2 moles of magnesium oxide, a 1:1 ratio.

Empirical formulae

To find it from masses or percentages: divide each element's mass (or percentage) by its $A_r$ to get moles, then divide all the results by the smallest to get the simplest ratio. If the answers are not whole numbers, multiply through to clear them.

Worked example

Examples in context

Example 1. Scaling up a reaction

A factory making lime heats limestone ($\text{CaCO}_3$) and uses reacting-mass calculations to work out how much lime each tonne of limestone yields. The mole method lets them plan raw-material orders and predict output exactly.

Example 2. Identifying an unknown

When chemists analyse a new compound, they measure the mass of each element and calculate the empirical formula, which is often the first clue to its identity. The same percentage-to-moles method used in the worked example is a routine analytical step.

Example 3. Checking a burning reaction

When a strip of magnesium burns, weighing it before and after shows a mass increase, which seems to break conservation of mass until you remember the magnesium has combined with oxygen from the air. Adding the mass of oxygen that joined accounts for the increase exactly, confirming that mass is conserved once every reactant and product, including gases, is included.

Working with the ratio carefully

The equation ratio is where most marks are won or lost. It is the ratio of the big numbers in front of the formulae, not the ratio of the masses. For $\text{N}_2 + 3\text{H}_2 \rightarrow 2\text{NH}_3$, one mole of nitrogen reacts with three moles of hydrogen to give two moles of ammonia, a 1:3:2 ratio in moles. To turn that into masses you must convert each through its own relative formula mass. Keeping moles and masses separate, and only using the ratio on the moles, is the habit that makes reacting-mass questions reliable, and it is exactly the skill extended into titration and gas-volume calculations in Unit 2.

Try this

Q1. State what is meant by the conservation of mass. [1 mark]

• Cue. The total mass of products equals the total mass of reactants.

Q2. A compound has moles of C and H in the ratio 1 : 3. Write its empirical formula. [1 mark]

• Cue. $\text{CH}_3$.