Reacting mass calculations
If you know the massThe amount of matter an object contains. Mass is measured in kilograms (kg) or grams (g). of a reactantA substance that reacts together with another substance to form products during a chemical reaction. and a productA substance formed in a chemical reaction., you can use simple ratioThe relative sizes of two or more numbers, shown as whole numbers. For example, 1:2 or 3:2. to calculate reacting masses and product masses.
Example one – reactant mass
Question
When 12 g of carbon is burned in air, 44 g of carbon dioxide is produced. What mass of carbon is needed to produce 11 g of carbon dioxide?
12 g of carbon makes 44 g of carbon dioxide.
12 ÷ 44 g of carbon will make 1 g of carbon dioxide.
Therefore, to make 11 g of carbon dioxide, you will need 11 × (12 ÷ 44 g) = 3 g of carbon.)
Alternative answer
An alternative method is as follows: Mass unknown = (Mass of known ÷ Mr known) × Mr unknown.
This works because (Mass of known ÷ Mr known) gives you the number of moles of the substance with the known mass. In this case:
(11 g ÷ 44) × 12 = 3 g of carbon to make 11 g of carbon dioxide.
Example two – product mass
Question
When 5.0 g of calcium carbonate is decomposeIf a substance decomposes, it breaks down into simpler compounds or elements. by heating, it produces 2.2 g of carbon dioxide. What mass of calcium carbonate is needed to produce 8.8 g of carbon dioxide?
5 g of calcium carbonate makes 2.2 g of carbon dioxide.
5 ÷ 2.2 g of calcium carbonate will make 1 g of carbon dioxide.
Therefore, to make 8.8 g of carbon dioxide, you will need 8.8 × (5 ÷ 2.2 g) = 20 g of carbon.
Alternative answer
An alternative method is as follows: Mass unknown = (Mass of known ÷ Mr known) × Mr unknown.
This works because (Mass of known ÷ Mr known) gives you the number of moles of the substance with the known mass. In this case:
(8.8 g ÷ 44) × 100 = 20 g of calcium carbonate to make 8.8 g of carbon dioxide.
More reacting mass calculations
The mass of a product or reactant can be calculated using the balanced equation. Follow these steps.
- Write out the balanced symbol equation. Underline the two substances you are interested in.
- Write the given mass of a substance under its formula.
- Work out the total relative formula mass (Mr) for each substance (the one you know and the one you are trying to find out). Write these under their formulae.
- Calculate: unknown mass = (known mass ÷ total Mr of known substance) × total Mr of unknown substance.
Worked example
Question
Calculate the mass of iron (Fe) that can be extracted from 8 g of Fe2O3 in the reaction with carbon.
| Step one | 2Fe2O3 + 3C → | 4Fe + 3CO2 |
| Step two | 8 g | ? g |
| Step three | 2 × 160 = 320 | 4 × 56 = 224 |
| Step four | mass of iron formed = (8 ÷ 320) × 224 = 5.6 g |
| Step one |
| 2Fe2O3 + 3C → |
| 4Fe + 3CO2 |
| Step two |
| 8 g |
| ? g |
| Step three |
| 2 × 160 = 320 |
| 4 × 56 = 224 |
| Step four |
| mass of iron formed = (8 ÷ 320) × 224 = 5.6 g |
