Finding the formula of a hydrated salt

IGCSE Chemistry

Finding the formula of a hydrated salt

The purpose of this experiment is to determine the empirical formula of a hydrate.

Hydrates are inorganic salts that contain a specific number of water molecules loosely attached.

An example is sodium carbonate decahydrate (washing soda). The formula for this substance is Na₂CO₃•10H₂O. The raised dot does not mean “multiply”; rather, it means that the water molecules are loosely attached.

Hydrates can normally be decomposed into the anhydrous (“without water”) salt and water by gentle heating according to the equation below. From the data collected the number of molecules of hydrated water will be determined per formula unit of anhydrous salt.

                                 CuSO₄•xH₂O  ®  CuSO₄ + xH₂O

What to do

1.  Determine the mass of an evaporating dish and record it to the nearest 0.01 g.

2.  Add 2.00 g of copper sulphate hydrate to the evaporating dish and record the combined mass of the

     salt and evaporating dish.

3.  Place the evaporating dish on the wire gauze on your tripod. Begin heating the sample slowly.

4.  After about 10 minutes of gentle heating, increase the heat until no further change in the salt is

     apparent.  At this point, the salt will seem to have changed from a crystalline solid to a grainy solid.

     You may need to use your glass stir rod to gently break up some of the crystals to assist in the

      evaporation of the water.

5.   Use tongs to place the evaporating dish on a heat proof mat in order to cool. Cover the salt with a

      watch glass to slow the possible re-absorption of water by the anhydrous salt.

6.   When the evaporating dish has cooled sufficiently to be handled, determine the mass of the

      evaporating dish and contents.

7.   Return the evaporating dish to the ring stand and re-heat it. Allow it to cool, then determine the mass

      of the evaporating dish with its contents again.

8.   Continue this process until a constant mass is attained.

9.   Record the lowest mass of the crucible and anhydrous salt.

Results

Design your own results table and record the raw data you have obtained.  Also record any observations you have made.

Calculations

1.    What is the mass of hydrated copper sulphate used in this experiment?

2.     What is the mass of anhydrous copper sulphate left at the end of the experiment?

3.     What is the mass of water contained in the hydrated copper sulphate?

4.     How many moles of anhydrous copper sulphate are left?

5.     How many moles of water were in the hydrated copper sulphate?

6.     What is the simplest whole number ratio of moles of anhydrous copper sulphate to water?

7.     What is the formula of hydrated copper sulphate?

Questions

1.  How do you know when the reaction is complete?

2.  How could you make this experiment more accurate?

3.  Give two specific safety measures when carrying out this experiment?