Reacting mass calculations

If you know the mass of a reactant and a product, you can use simple ratios 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 decomposed 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.

  1. Write out the balanced symbol equation. Underline the two substances you are interested in.
  2. Write the given mass of a substance under its formula.
  3. 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.
  4. 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 one2Fe2O3 + 3C →4Fe + 3CO2
Step two8 g? g
Step three2 × 160 = 3204 × 56 = 224
Step fourmass of iron formed = (8 ÷ 320) × 224 = 5.6 g