Analía Bellizzi – Chemistry Classes

Ronald Reagan Senior High School

Finding the Formula for a Hydrate

Finding the Formula for a Hydrate lab


  • Hot plate
  • Beaker
  • Triple beam balance
  • Glass rod
  • 2 to 2.5 g CuSO45H2O
  • 2 to 3 g MgSO4XH2O

Special Notes:
The masses you will be measuring must be as accurate as possible. Try not to spill any chemical and DO NOT remove the glass rod from the beaker at any moment. Some of the crystals could be attached to the glass rod and this will ruin your experiment.

Procedure: Part I

  1. Turn on your hot plate.
  2. Be sure your beaker and glass rod with rubber policeman does not have any crack, and that that they are clean and dry.
  3. Measure the mass of the empty beaker with the glass rod inside (GLASS SIDE INSIDE THE BEAKER AND RUBBER POLICEMAN AS HANDLE). Record this value in your data table with the maximum available precision.
  4. Obtain about 1 to 2 g of CuSO4●5H2O in your beaker and weigh it, together with the glass rod. Record this value in your data table with the maximum available precision.
  5. Calculate the mass of the CuSO4●5H2O by difference between your second and your first measurement.
  6. Place your beaker with the sample and the rod on the hot plate USING A WIRE GAUZE to protect the beaker from breaking.
  7. Heat the compound gently Note the release of any steam from the beaker.
  8. Continue heating gently until the salt turns completely light grey. Be careful not to overheat! The heat can become so intense that the sulfate in the salt begins to break down. If this happens the salt will turn yellow and produce a sulfurous smell. It will also ruin your data since it will reduce the mass more than expected due to the decomposition of the salt.
  9. Use the glass rod to stir the chemical to avoid overheating in some areas. Use the glass end to stir the compound.
  10. Stop heating when the salt has lost all traces of blue color. If the compound turn yellow, remove it from the hot plate and place it on a ceramic pad (DO NOT PLACE THE HOT BEAKER ON THE COOL TABLE, IT CAN SHATTER) Allow the beaker and its contents to cool completely.
  11. Once the beaker is cool, measure the mass of the beaker, the anhydrous salt and the glass rod.
  12. Heat the contents again for a short time (3 minutes). Then allow it to cool and weigh it. If the mass is the same as the previous weighing, then the salt has been completely dehydrated. If not, repeat the heating/cooling/weighing process until two successive weighing have the same mass.
  13. Record the final mass of the anhydrous salt in you lab notebook and do the calculations to show that the molar ratio of water to anhydrous salt really is 5:1.
  14. Empty the anhydrous salt onto a large watch glass. Use the dropper to add a very little water to the anhydrous copper (II) sulfate.
  15. Describe what happens in your lab notebook. For your report explain what is happening at the molecular level when you add water.
  16. When you finished this part of the lab empty all the CuSO4 into the beaker provided by your instructor for this purpose.

Part II

In this part of the lab you will repeat the same procedure performed for the salt of known formula with a salt for which you do not know the hydrate formula. The salt is magnesium sulfate – MgSO4 and, the same as Copper sulfate, it exists as a hydrate, but in this case we will find the amount of water surrounding the compound. (MgSO4●XH2O).
Use a minimum of 2 g. This will help to reduce errors due to small lab balance inaccuracies.
An additional challenge is that both the hydrate and anhydrous salt are white.
Finally, unless you frequently stir the crystals they will combine and harden, possibly trapping water inside To prevent stir continuously. You can use a metallic spatula this time. If clumps are formed, just continue stirring and heating until you see

Formula of a hydrate lab data table1
Formula of a hydrate lab data table2

Analysis questions:

  1. What was the expected mass of anhydrous copper (II) sulfate? The expected mass of water to be lost by heating?
  2. How well did your prediction match up with your results for copper (II) sulfate pentahydrate?
  3. Say that your mass of water lost was too low compared with your prediction. What may have caused this? Describe a scenario as it could really have happened in the lab.
  4. Say that your mass of water lost was too large compared with your prediction. What may have caused this? Describe a scenario as it could really have happened in the lab.
  5. What is the formula of the hydrate of magnesium sulfate? What is the name of this hydrate?
  6. What specific sources of error could lead to a result in which you come up with a formula with too few moles of water of hydration compared with the correct formula? Look up the correct hydrate formula online.
  1. Calculate the mass percent of water in your original hydrate sample using the mass of water lost and the mass of the blue hydrate.
    The accepted value is 36.1%. What is your percent error? Were you successful in converting the hydrate completely to the anhydride?
  2. Draw a particulate model of the blue hydrate and the anhydride using a diamond to represent the anhydride and a circle to represent H2
  3. Use the pictures in Question 2 to explain your observations in Step 8.
  4. If the color of the hydrate were identical to the color of the anhydride, how could you be sure that you had driven off all the water from the hydrate?

Making Anhydrous Magnesium Sulfate from Magnesium Sulfate Heptahydrate (Epsom Salts)

MgSO4 • 7H2O = Magnesium Sulfate Heptahydrate (Epsom Salts)
MgSO4 = Anhydrous Magesium Sulfate

Anhydrous Magnesium Sulfate will absorb water from the atmosphere, acting as a very effective desiccant or drying agent.
The drying power of Anydrous Magnesium Sulfate is superior to that of Anhydrous Calcium Sulfate (Drie-Rite).
Epsom Salts can be easily converted to Anhydrous Magnesium Sulfate by baking in an oven at temperatures between 400°F and 482°F

150°C = 302°F   At this temperature Magnesium Sulfate Heptahydrate is converted to Magnesium Sulfate Monohydrate
200°C = 392°F   At this temperature Magnesium Sulfate Monohydrate is converted to Anhydrous Magnesium Sulfate
250°C = 482°F   Loses all waters of hydration. yielding Anhydrous Magnesium Sulfate
1124°C = 2055°F   Decomposes to MgO, SO2, SO3 before or as it reaches melting point.

MgSO4 7H2O = 246.4564 molecular weight
MgSO4 anhydrous = 120.36 molecular weight

120.36 / 246.45 = 48.83%
246.45 / 120.36 = 204.76%

Anhydrous MgSO4 can absorb 204.7% it’s weight in water

Your lab should include: