Reacting Masses

Chemical equations can be used to calculate the reacting masses of reactants and products
The mass ratio between the substances is identified using the balanced chemical equation
The steps are:
1. Write down the balanced equation for the reaction
2. Write the relative formula masses under the substances and add the units in the question
3. Multiply the relative formula masses by the coefficients in the equation
4. Find the mass of product for 1 g of reactant
5. Scale up for the mass given in the question

Worked example

Example 1

Calculate the mass of magnesium oxide that can be made by completely burning 6.0 g of magnesium in oxygen.

Answer:

Step 1: Write the balanced equation

2Mg (s) + O₂ (g) ⟶ 2MgO (s)

Step 2: Add RFMs and units

24 g

40 g

Step 3: Multiply by coefficients

2 x 24 = 48 g

2 x 40 = 80 g

Step 4: Cross multiply for 1 g

1 g	80 / 48 = 1.66 g

Step 5: Scale up to mass in question

6 g

6 x 1.66 = 10 g

Worked example

Example 2

Calculate the mass of aluminium, in tonnes, that can be produced from 51 tonnes of aluminium oxide.

Answer:

Step 1: Write the balanced equation

2Al₂O₃ ⟶ 4Al + 3O₂

Step 2: Add the RFMs and units

102 tonnes

27 tonnes

Step 3: Multiply by coefficients

2 x 102 = 204 tonnes

4 x 27 = 108 tonnes

Step 4: Cross multiply for 1 tonne

1 tonne

108 / 204 = 0.53 tonne

Step 5: Scale up to mass in question

51 tonnes

51 x 0.53 = 27 tonnes of Al

Examiner Tip

As long as you are consistent it doesn't matter whether you work in grams or tonnes or any other mass unit as the reacting masses will always be in proportion to the balanced equation.

Percentage Composition

The percentage by mass of an element in a compound can be calculated using the following equation:

$% mass of an element = \frac{A_{r} \times number of atoms of the element}{M_{r} of the compound} \times 100$

Worked example

Calculate the percentage by mass of calcium in calcium carbonate, CaCO₃.

Answer:

Calculate % Mass of an Element in a Compound WE, downloadable IGCSE & GCSE Chemistry revision notes

Examiner Tip

Don’t forget to multiply your answer by 100 in order to convert it to a percentage.

Reacting Masses (Edexcel GCSE Chemistry: Combined Science)

Revision Note

Reacting Masses

Worked example

Worked example

Examiner Tip

Percentage Composition

Worked example

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Author: Stewart