# 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 CaCO_{3 }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:

CaCO_{3 }40 + 12 + (3 x 16) = 100

CaO 40 + 16 =56

Calculate the number of moles of CaCO_{3 }used.

**n=mass/Mr 50/100 = 0.5 mol**

From the equation we can see that one mole of CaCO_{3} produces one mole of CaO.

Therefore 0.5 mol of CaCO_{3} 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 CaCO_{3 }produces 28g CaO

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

*I**f 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.*