#### Calculating Reacting Masses

Reacting Masses: Chemical equations can be used to calculate the moles or masses of Reactants and Products

• Use information from the question to find the mole of a substance / reactant
• Identify the ratio of the substance and reactants and find the moles of others
• Apply mole calculations to find answer

Example 1:

Calculate the Mass of Magnesium Oxide that can be made by completely burning 6 g of Magnesium in Oxygen

Magnesium (s)  + Oxygen (g)  →   Magnesium Oxide (s)

Symbol Equation:     2Mg     +   O2  →       2MgO

Relative Formula Mass:    Magnesium : 24             Magnesium Oxide : 40

Step 1 – Calculate the moles of Magnesium Used in reaction

Moles = Mass ÷ Mr                                       Moles = 6 ÷ 24 = 0.25

Step 2 – Find the Ratio of Magnesium to Magnesium Oxide using Chemical Equation

Magnesium               Magnesium Oxide

Mol                            2                                      2

Ratio                         1                                      1

Mol                          0.25                                 0.25

Moles of Magnesium Oxide = 0.25

Step 3 – Find the Mass of Magnesium Oxide

Moles of Magnesium Oxide = 0.25

Mass = Moles x Mr                                       Mass = 0.25 x 40 = 10

Mass of Magnesium Oxide Produced = 10 g

Example 2:

Calculate the Mass, in Tonnes, of Aluminium that can be Produced from 51 Tonnes of Aluminium Oxide

Aluminium Oxide (s)  →   Aluminium (s)   +   Oxygen (g)

Symbol Equation:     2Al2O3     →       4Al       +       3O2

Ar and Mr:                   Aluminium : 27    Oxygen : 16    Aluminium Oxide : 102

1 Tonne = 106 g

Step 1 – Calculate the moles of Aluminium Oxide Used

Mass of Aluminium Oxide in Grams = 51 x 106 = 51,000,000  g

Moles = Mass ÷ Ar                             Moles = 51,000,000 ÷ 102 = 500,000

Step 2 – Find the Ratio of Aluminium Oxide to Aluminium using the balanced Chemical Equation

Aluminium Oxide         Aluminium

Mol                            1                                     2

Ratio                         1                                     2

Mol                      500,000                         1,000,000

Moles of Aluminium = 1,000,000

Step 3 – Find the Mass of Aluminium

Moles of Aluminium = 1,000,000

Mass in grams = Moles x Ar                Mass = 1,000,000 x 27 = 27,000,000

Mass in Tonnes = 27,000,000g ÷ 106 = 27 Tonnes

Mass of Aluminium Produced = 27 Tonnes

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### Author: Jamie

Jamie got a First class degree in Chemistry from Oxford University before going on to teach chemistry full time as a professional tutor. He’s put together these handy revision notes to match the Edexcel IGCSE Chemistry specification so you can learn exactly what you need to know for your exams.