Formula of a Hydrate
Discussion:
The purpose of this experiment is to determine the empirical formula of a hydrate. Hydrates are
inorganic salts which contain a specific number of water molecules loosly attached. Examples
are: magnesium sulfate heptahydrate (epsom salts) and sodium carbonate decahydrate (washing
soda). The formulas for these substances are MgSO 4●7H2O and Na2CO3●10H2O. They can also
be represented as MgSO4(H2O)7 and Na2CO3(H2O)10. Not all hydrates have simple formulas like
these. For example, a hydrate of cadmium sulfate seems to have 2.66 molecules of water for
each molecule of CdSO4. This hydrate is best represented as (CdSO4)3(H2O)8. When
determining the formula of a hydrate you must not assume that it is one with a simple formula.
Hydrates can normally be decomposed into the anhydrous salt and water by gentle heating. From
the data collected the number of molecules of water will be determined per formula unit of
anhydrous salt.
Safety Considerations:
Exercise extreme caution when using Bunsen burners.
Equipment Needed:
Bunsen burner with hose
Evaporating dish
Clay triangle
Iron ring
Ring stand
Crucible tongs
Triple-beam balance
Calculator and pencil
Materials Needed:
Hydrate sample: CuSO4●xH2O
Procedure:
1. Clean and dry an evaporating dish. Evaporating dishes are very fragile, so handle yours
with extreme caution.
2. Mass the empty evaporating dish on a triple-beam balance. Record the mass to the
nearest 100th of a gram (you will have to estimate the last digit). You will lose points if
your mass measurements do not contain two decimal places.
3. Add about 2 grams of the blue copper(II) sulfate hydrate to the evaporating dish. Find
the mass of the evaporating dish and copper(II) sulfate hydrate to the nearest 100 th of a
gram.
4. Calculate the mass of the copper(II) sulfate hydrate in the dish. It does not have to be
exactly 2.00 grams.
5. Clamp the iron ring to the ring stand. Place a clay triangle on the iron ring and carefully
place the evaporating dish in the triangle.
6. Light a Bunsen burner and heat the dish for 10 minutes. The copper(II) sulfate hydrate
will turn from blue to white as the water is driven off. If blackening occurs, reduce the
heat.
7. Allow the dish to cool, then mass the evaporating dish and anhydrous copper(II) sulfate
to the nearest 100th of a gram.
8. Heat the evaporating dish again for 5 minutes. Allow it to cool and find its mass to the
nearest 100th of a gram.
9. If this mass agrees with the previous mass to within 0.01 grams, no further heating is
needed. If the mass has changed by more than 0.01 grams, heat the dish for another
five minutes. Repeat this action until the mass is constant. This ensures that the
reaction has finished.
10. Calculate the mass of the anhydrous CuSO4 left in the evaporating dish.
11. Calculate the mass of water lost during heating.
12. Convert the mass of anhydrous CuSO4 to moles of CuSO4. The molar mass of CuSO4 is
159.610 g/mol.
13. Convert the mass of water lost to moles of water. The molar mass of water is 18.0153
g/mol.
14. Determine the value of x in the formula of the original hydrate: CuSO4●xH2O
15. Add a few drops of water to the anhydrous CuSO4. Observe what happens to it.
16. You may dispose of all chemicals in the sink with running water.
Data and Observations:
Mass of evaporating dish: __________ g
Mass of evaporating dish + CuSO4●xH2O: __________ g
Mass of CuSO4●xH2O: __________ g
Mass of evaporating dish + CuSO4 (after 1st heating): __________ g
Mass of evaporating dish + CuSO4 (after 2nd heating): __________ g
Mass of evaporating dish + CuSO4 (after 3rd heating, if necessary): __________ g
Mass of evaporating dish + CuSO4 (after 4th heating, if necessary): __________ g
Mass of evaporating dish + CuSO4 (after 5th heating, if necessary): __________ g
Mass of CuSO4: __________ g
Mass of water lost: __________ g
Moles of CuSO4: __________ mol
Moles of H2O lost: __________ mol
Formula of hydrate: CuSO4●____H2O
Post-Lab Questions:
1. If you didn’t drive out all of the water during heating, what effect would that have on the
empirical formula you worked out? Be specific.
2. Another hydrate, MgSO4●7H2O, is often used for this experiment. If you started with 2.15
grams of MgSO4●7H2O, how many grams of MgSO4 should be left after all the water is
driven out?