Many ionic compounds contain, as part of their composition, molecules of water. These molecules often hydrate the ions in the lattice. Most students are familiar with copper sulfate, which, when heated, loses water of crystallisation and becomes white / grey; the product can be used as a test for the presence of water, since only water will restore the original blue colour to the compound.
The formula of such a compound is written in the form CuSO4xH2O, where x is usually a small whole number.
Since heat will usually drive off the water, there is an accompanying mass loss, corresponding to the mass of water originally present. The mass loss can be used to find the value of x and hence the full formula of the compound.
In this activity, we carry out gravimetric analyses of three compounds to determine their full formulae.
- Open Yenka file Model 1. The display shows a beaker, containing 10 g of hydrated barium chloride on a tripod stand. It can be heated using a Bunsen burner, which can be turned on using the control at its right-hand side. Turn on the Bunsen burner and heat up the hydrated barium chloride. In the information panel you will see that the hydrated barium chloride becomes barium chloride and steam. Eventually, there is no hydrated compound left. The final product is barium chloride. What mass of barium chloride forms?
- Calculate the mass of water that must have been present in the original compound.
Answer10 − 8.525 = 1.475 g
- (a) Work out the formula for barium chloride.
(b) Work out the mass of 1 mole of barium chloride. Relative atomic masses are available on the Periodic Table which can be accessed using the relevant icon in the main toolbar.Answer(a) BaCl2
(b) 137 + (2 × 35.5) = 208 g
- (a) Calculate the number of moles of barium chloride remaining after heating.
(b) Calculate the number of moles of water lost.Answer(a) 8.525 / 208 = 0.041
(b) 1.475 / 18 = 0.082
- By examining the ratio between these numbers, decide on the value of x in the formula BaCl2xH2O.
Answerx = 2, so the formula is BaCl2 2H2O.
- Now, open the scene called 'Copper sulfate'. Heat the hydrated copper sulfate until there is no further mass loss.
(a) What is the final mass of anhydrous copper sulfate?
(b) What mass of water has been lost?Answer(a) 6.392 g
(b) 3.608 g
- (a) Work out the formula for copper sulfate.
(b) Work out the mass of 1 mole of copper sulfate.
(c) Calculate the number of moles of copper sulfate remaining after heating.
(d) Calculate the number of moles of water lost.
(e) By examining the ratio between these numbers, deduce the full formula for hydrated copper sulfate.Answer(a) CuSO4
(b) 63.5 + 32 + (4 × 16) = 159.5 g
(c) 6.392 / 63.5 = 0.04
(d) 3.608 / 18 = 0.20
(e) CuSO4 5H2O
- Now, open the scene called 'Magnesium sulfate'. Heat the hydrated magnesium sulfate until there is no further mass loss. Use the results to deduce the full formula for hydrated magnesium sulfate.
The complete formula of a hydrated ionic compound can be found by determining the mass loss of the compound on heating. From the mass loss, the moles of water initially present can be found. From the mass remaining, the moles of anhydrous compound can be found. Comparison of the ratio of these figures allows the formula to be deduced.