Calculating masses in reactions – 3 important steps
- Write out a balanced equation
- Work out the Mr
- Divide to get one and multiply to get all
- g. What mass of magnesium oxide is produce when 60g of magnesium is burnt in air
- Step 1: Write a balanced equation = 2Mg + O2 = 2MgO
- Step 2: 2MgO (Mg = 24 O = 16) 48 + 32 = 80
- 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
- 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
- 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)
- 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