1.25 calculate reacting masses using experimental data and chemical equations

Worked Example: Magnesium oxide –

Q) Work out the mass of magnesium oxide that can be made by burning 6g of magnesium in oxygen completely 2Mg + O2 —> 2MgO 

Method 1: Using moles 

  1. Calculate the moles of magnesium reacted – Ar of Mg = 24. Amount = 6/24 = 0.25 mol
  2. Calculate the amount of magnesium oxide formed – 2 mol Mg —> 2 mol MgO
  3. Calculate the mass of Magnesium Oxide formed – Mr of MgO = 24 + 16 = 40. Mass of MgO = 40 x 0.25 = 10g of Mgo 

Method 2: Using mass ratios 

2Mg + O2 —> 2MgO 

Ar of Mg = 24. Mr of MgO = 40

  1. 2 (2 Mg molecules) x 24 = 48g of Mg —> 2 x 40 = 80g og MgO
  2. 6 Mg —> (6 / 48) +80 = 10g of MgO 
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