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Calculating masses in reactions – 3 important steps

Calculating masses in reactions – 3 important steps

  1. Write out a balanced equation
  2. Work out the Mr
  3. Divide to get one and multiply to get all
    1. g. What mass of magnesium oxide is produce when 60g of magnesium is burnt in air
    2. Step 1: Write a balanced equation = 2Mg + O2 = 2MgO
    3. Step 2: 2MgO (Mg = 24 O = 16) 48 + 32 = 80
    4. Step 3: Find the ratio of Mg to the total : 48 : 80 divide by 48 (1: 1.667)  times by 60g = 60:100

Therefore 60g of magnesium gives you 100g of magnesium oxide

Percentage Yield:

  • Compares your actual yield from your experiment against your calculated yield
  • Percentage yield = actual yield in grams / predicted yield in grams } all times 100
  • In experiments percentage yield is always lower than 100% because something always is lost along the way
    1. If the reaction is reversible – because the reactants are not always completely reacted – the reaction is reversible so once something converts one way it immediately goes back the other so it is never at the same state as the same time
    2. If you have to filter the experiment then you never quite get everything through (you either lose some of the liquid or some of the solid)
    3. Unexpected reaction – because not everything is completely pure in the real world then always some little other things in the reactant use up some of the reactants
  • Product Yield is important for sustainable development
    • The highest amount of yield at the lowest energy is the most sustainable
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