Reacting Quantities

The numbers in a balanced equation give the ratio of the amount of each substance in the reaction.

We can use this information to calculate quantities of reactants or products.

50g of CaCO₃ are heated how much CaO will be formed

First write a balanced equation: CaCO_3(s) → CaO_(s) + CO_2(g)

Then calculate the Mr of the substances we are interested in:

CaCO₃            40 + 12 + (3 x 16) = 100

CaO                40 + 16 =56

Calculate the number of moles of CaCO₃ used.

n=mass/Mr                          50/100 = 0.5 mol

From the equation we can see that one mole of CaCO₃ produces one mole of CaO.

Therefore 0.5 mol of CaCO₃ produces 0.5 mol of CaO.

Finally calculate the mass of 0.5 mol of CaO:

n=mass/Mr,                      mass=Mr x n,                           0.5 x 56 = 28g

Therefore 50g of CaCO₃ produces 28g CaO  

The secret of success in A-level chemistry calculations is to write everything down carefully.

If you can write it out in words, there is a good chance that you can actually understand it.

If you can understand it, the examiner will also be able to understand it and might be able to give you some credit for a partial answer.