Example Question - 4

Question: A sample of hydrated magnesium sulfate, MgSO₄·xH₂O, has a mass of 2.46 g. It is heated in a crucible until its mass stops changing. The anhydrous MgSO₄ left has a mass of 1.20 g. Find x. (M_r: MgSO₄ = 120, H₂O = 18)Water of crystallisation is water that is chemically bonded within the crystal lattice of a salt. Heating drives it off, leaving the anhydrous salt.Step 1: mass of water lost.2.46 - 1.20 = 1.26 gStep 2: moles of each substance.n(MgSO₄) = 1.20 ÷ 120 = 0.0100 mol
n(H₂O) = 1.26 ÷ 18 = 0.0700 molStep 3: ratio. Divide by the smaller value.x = 0.0700 ÷ 0.0100 = 7Answer: the formula is MgSO₄·7H₂O.Heating to constant mass means heating, cooling and weighing repeatedly until two readings agree. This makes sure all the water has been removed. If some water remained, the mass of water lost would be too small and the calculated value of x would be too low.A lid on the crucible, lifted slightly, helps stop solid being lost by spitting. If solid is lost, the mass lost is too large and x comes out too high.