So first you work out the ratio. The formula tells us that for every one 'amount' of Ca and Cthere will be 3x the amount of oxygen. So, the ratio is 1:1:3, (=5) therefore 70g divided by 5 = 14.
If 70g of CaCO3 is heated there will be (1x14) 14g Ca, (1x14) 14g C and (3x14) 42g of O.
Weight of CaO= 42+14, 56g
(I hope this is right! Good luck xx)
