11.25g of hydrated copper (II) sulfate is heated to constant mass. The new mass is found to be 7.19g. What is the formula of the hydrated copper (II) sulfate?
Hydrated Copper (II) sulfateThe hydrated term means that this compound contains water of crystallisationWater of crystallisation means that there are chemically bonded water molecules withing the compound that is namedWhen we heat hydrated copper (II) sulfate, the constant mass achieved means that we have caused all these water molecules to evaporateAs there is no more water left to evaporate, the mass of the copper (II) sulfate is constant
Before we heated the compound the mass was 11.25g. After heating to constant mass the mass was 7.19g meaning the mass of water lost was: 11.25-7.19 = 4.06g of waterSo essentially the 11.25g of hydrated copper (II) sulfate contained 7.19g of copper sulfate with 4.06g of chemically bonded water molecules.However in order to get the formula of the compound we know we have to use moles:
Moles = mass(g)/relative formula massWe find the relative formula mass using the mass numbers of the elements found in the periodic table
Moles of copper (II) sulfate (CuSO4) = 7.19g/159.5 = 0.04508 moles of copper (II) sulfateMoles of water (H2O) = 4.06/18 = 0.2256 moles of water
We therefore know that the ratio of copper (II) sulfate to water is:Copper (II) sulfate : Water0.04508 : 0.2256
We then divide both numbers by the moles of copper (II) sulfate to find out how many moles of water there are for every 1 mole of copper (II) sulfate:
0.04508/0.04508 : 0.2256/0.04508 = 1 : 5
This means for every 1 mole of hydrated copper (II) sulfate there are 5 moles of chemically bonded water of crystallisation:This formula is written as:CuSO4.5H2O
