Mole Calculations

On Friday we practised balancing equations and working out formula masses. This will allow us to calculate the number of moles, and the masses, of reactants and products using the equations:




Here is an example which we will go through at the start of the lesson, and which will be easy for you if you've read this first:

What mass of calcium hycroxide do we need to neutralise 146g of hydrochloric acid?

First we need a balanced symbol equation:
2HCl + Ca(OH)2   >  CaCl2 + 2H2O

This tells us that one mole of calcium hydroxide will react with two moles of HCl (because the balanced equation has 2HCl but only one Ca(OH)2 )


So how many moles of HCl do we have? This is the mass divided by the molar mass, so 146 / (1+35/5) = 4 mol.

So how many moles of Ca(OH)2 do we need? One will react with two moles of HCl, so we will need 2 to react with 4.


What is the mass of Ca(OH)2 needed? This is given by mass = moles x formula mass = 2 x (40+(2 x (16+1))) = 2 x 74 = 148g