EXPERIMENT 4B
The Empirical Formula of a Hydrate
INTRODUCTION:
Many ionic compounds (salts) which have been crystallized from a water
solution appear to be perfectly dry, yet when heated yield large quantities of
water. The crystals change form, even color sometimes, as the water is driven off.
This suggests that water was present as part of the crystal structure. Such
compounds are called hydrates. The number of moles of water present per
mole of anhydrous salt (anhydrous means with out water, or dry) is usually some
simple whole number.
An example of a hydrate is copper sulfate pentahydrate,
CuSO4-5H2O. Thus there are 5 molecules of water for every formula unit of the
anhydrous CuSO4
In this experiment you will be given a hydrate. You will determine the
mass of water driven off by heating and the amount of anhydrous salt which
remains. Given the molar mass of the anhydrous salt, you can compute the moles
of anhydrous salt and moles of water. This ratio will give you the Empirical
formula of the hydrate.
You will find the empirical formula of one of the following hydrated salts:
Barium chloride
Sodium carbonate
Calcium sulfate
Magnesium sulfate
Zinc sulfate (Zn2+)
Manganese sulfate (Mn2+)
PROCEDURE:
1. Place a clean, dry crucible on a clay triangle mounted on an iron ring attached
to your ring stand. Heat the crucible with a Bunsen burner for two or three
minutes. Remove the flame.
2. When the crucible is cool enough to touch, find the mass of the crucible to the
nearest 0.001 g.
3. Put enough of the hydrate crystals in the crucible to fill it one-fourth full.
Weigh the crucible to nearest 0.001 g.
4. Place the crucible and contents back on the ring stand and heat gently.
Gradually increase the flame until the crucible bottom is at most a dull red.
Maintain this temperature for five minutes.
5. Remove the burner and allow to cool until the crucible is cool enough to touch.
6. Determine the mass of the crucible and anhydrous salt to the nearest 0.001 g.
7. Repeat steps 4-6 until your last reading agrees within +/- 0.005 g of the
previous weighing.
8. Dispose of your chemical in the inorganic waste container found in the hood.
Clean and return the crucible.
PROCESSING THE DATA:
1. Calculate the mass of anhydrous salt.
2. Calculate the number of moles of anhydrous salt you used.
3. Calculate the mass of the water driven from the hydrate.
4. Calculate the number of moles of water driven from the hydrate.
5. Calculate the moles of water per mole of anhydrous salt. This will be a
decimal ratio. Round it to the nearest whole number.
6. Write the empirical formula for your hydrate.