Theoretical Masses of Products (AQA GCSE Chemistry)

In an experiment to displace copper from copper(II)sulfate, 6.5g of zinc was added to a solution of copper(II)sulfate.

The balanced equation for the reaction is: 

Zn (s) + CuSO₄ (aq)  ⟶ Cu (s) + ZnSO₄ (aq)

The copper was filtered off, washed, dried and weighed. the final mass of copper obtained was 4.8 g.

Calculate the percentage yield of copper.

Answer:

• Step 1: Calculate the moles of zinc that reacted:

  • Moles = mass / A_r 

  • Moles = 6.5 / 65 = 0.1 mol

• Step 2: Calculate the maximum number of moles of copper that could be formed:

  • From the molar ratio, one mole of zinc reacts to form one mole of copper

  • So, 0.1 moles of zinc will form 0.1 moles of copper

• Step 3: Calculate the maximum mass of copper that could be formed:

  • Mass = moles x A_r 

  • Mass = 0.1 x 63.5 = 6.35 g

• Step 4: Calculate the percentage yield of the reaction:

  • Percentage yield =  x 100

  • Percentage yield =  = 76 %

The key to success in all calculations is to have a methodical step-by-step approach and show all your workings. Be careful when working out the molar masses not to include coeffiecients. It is very easy to make the mistake of calculating the molar mass of magnesium oxide as 80 g instead of 40 g, when working from an equation:

2Mg (s) + O₂ (g) → 2MgO (s)

(Mg = 24 g mol⁻¹; O = 16 g mol⁻¹ )

Remember that molar masses are calculated from chemical formulae, not from equations