Calculate Reacting Masses (Edexcel IGCSE Chemistry (Modular))

Worked Example

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 =  =  = 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

Worked Example

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 =  =  = 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 =  = 27 tonnes

Exam Tip

• When you see any of the following phrases in a question, make sure you work methodically through the process of calculating moles, using molar ratios and calculating mass

  • Calculate the mass...

  • Calculate the minimum mass...

  • Calculate the maximum mass...

• 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

Worked Example

A student reacts 1.2 g of carbon with 16.2 g of zinc oxide. The resulting products are 4.4 g of carbon dioxide and 13 g of zinc.

Determine the balanced equation for the reaction.

Answer: 

• Step 1: Write the unbalanced chemical equation:

  • C + ZnO → CO₂ + Zn

• Step 2: Write down the masses of each substance:

  • C = 12

  • ZnO = 65 + 16 = 81

  • CO₂ = 12 + (2 x 16) = 44

  • Zn = 65

• Step 3: Calculate the moles of each substance:

  • C = 1.2 / 12 = 0.1

  • ZnO = 16.2 / 81 = 0.2

  • CO₂ = 4.4 / 44 = 0.1

  • Zn = 13 / 65 = 0.2

• Step 4: Deduce the whole number ratio of substances:

  • C : ZnO : CO₂ : Zn

  • 0.1 : 0.2 : 0.1 : 0.2

  • 1 : 2 : 1 : 2

• Step 5: Apply the whole number ratio to the unbalanced equation:

  • 1C + 2ZnO → 1CO₂ + 2Zn

  • C + 2ZnO → CO₂ + 2Zn

Exam Tip

These questions look hard but they are actually quite easy to do, as long as you follow the steps and organise your work neatly.

Remember the molar ratio of a balanced equation gives you the ratio of the amounts of each substance in the reaction.