Reacting Masses (Oxford AQA IGCSE Combined Science Double Award)

Revision Note

Written by: Alexandra Brennan

Reviewed by: Stewart Hird

Chemical / symbol equations can be used to calculate:
- The moles of reactants and products
- The mass of reactants and products
To do this:
- Information from the question is used to find the amount in moles of the substances being considered
- Then, the ratio between the substances is identified using the balanced chemical equation
- Once the moles have been determined they can then be converted into grams using the relative atomic or relative formula masses

Magnesium undergoes combustion to produce magnesium oxide.

The overall reaction that is taking place is shown in the equation below.

2Mg (s) + O₂ (g) ⟶ 2 MgO (s)

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

A_r(O) = 16 A_r(Mg) = 24

Answer:

Step 1 - calculate the moles of magnesium
- Moles = $(\frac{mass}{M_{r}})$ = $(\frac{6}{24})$ = 0.25
Step 2 - use the molar ratio from the balanced symbol equation
- 2 moles of magnesium produce 2 moles of magnesium oxide
- The ratio is 1 : 1
- Therefore, 0.25 moles of magnesium oxide is produced
Step 3 - calculate the mass of magnesium oxide
- Mass = moles x M_r = 0.25 moles x (24 + 16) = 10 g

In theory, aluminium could decompose as shown in the equation below.

2Al₂O₃ ⟶ 4Al + 3O₂

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

A_r(O) = 16 A_r(Al) = 27

Answer:

Step 1 - calculate the moles of aluminium oxide
- Mass = 51 tonnes x 10⁶ = 51 000 000 g
- Moles = $(\frac{mass}{M_{r}})$ = $(\frac{51000000}{102})$ = 500 000
Step 2 - use the molar ratio from the balanced symbol equation
- 2 moles of aluminium oxide produces 4 moles of aluminium
- The ratio is 1 : 2
- Therefore, 2 x 500 000 = 1 000 000 moles of aluminium is produced
Step 3 - calculate the mass of aluminium
- Mass = moles x M_r = 1 000 000 moles x 27 = 27 000 000 g
- Mass in tonnes = $(\frac{27000000}{10^{6}})$ = 27 tonnes

Yield is the term used to describe the amount of product you get from a reaction
Even though no atoms are lost or gained in a chemical reaction, it is not always possible to obtain the calculated amount of product
This is because:
- Some reactants may be left behind in the equipment
- The reaction may be reversible and in these reactions a high yield is never possible as the products are continually turning back into the reactants
- Some products may also be lost during separation and purification stages such as filtration or distillation
- Some of the reactants may react in ways different from the expected reaction such as a substance reacting with an impurity in one of the reactants

Make sure you can give at least two reasons why you don't always obtain the expected amount of product for a simple recall style question.

Last updated: 27 March 2024

the (exam) results speak for themselves:

Did this page help you?