Finding the formula of hydrated copper(II) sulfate crystals
Wear eye protection at all times. Copper(II) sulfate(VI) is harmful if swallowed and irritates the eyes and skin.
Apparatus (per student)
• fresh copper(II) sulfate(VI) crystals • spatula • evaporating basin • pipeclay triangle • glass stirring rod and tongs
• Bunsen burner, tripod and heatresistant mat • access to accurate balance (at least to 2 decimal places) • eye protection
1 Weigh the evaporating basin and the mass of crystals (HARMFUL) that you have been allocated. 2 Record your results in a suitable table. 3 Place the pipeclay triangle and the evaporating basin containing your crystals on the tripod. 4 Very gently heat your crystals – a low Bunsen flame should be used for this. 5 Whilst heating the crystals, stir them using the glass stirring rod. At the same time grip the evaporating basin using the tongs, to prevent the evaporating basin toppling over and spilling. 6 At first the copper(II) sulfate(VI) will be ‘sticky’ but after a while it should not cling to the glass rod and will become powdery. 7 The colour of the copper(II) sulfate(VI) will change from blue to light blue to grey. It will not be white at the end. 8 When it gets to this stage, weigh the evaporating basin and anhydrous copper(II) sulfate(VI). Then reheat it for a short while and reweigh it. If constant mass is obtained, then all the water of crystallisation has been driven off from the crystals. 9 Record your result. 10 Calculate the mass of water and of anhydrous copper(II) sulfate(VI), and pool your results with those of the rest of the group. 11 Plot a graph of mass of anhydrous copper(II) sulfate(VI) (horizontal axis) against mass of water of crystallisation (vertical axis). From this calculate the mass of water that combines