Name ___________________________________________ Block ____________Date __________________
Chemistry Lab
Hydrated Crystals
Background
Many compounds are formed in reactions that take place in water solutions. The water is then
evaporated to obtain the crystalline compound. In some cases water molecules are weakly attracted to the ions
or molecules that make up the compound and are retained within the crystal structure. Crystalline compounds
that retain water during evaporation are referred to as being hydrated or are said to contain water of hydration.
The ratio of moles of water to moles of compound is a small whole number. The formula for the hydrated
compound nickel (II) sulfate is:
NiSO4 * 7H2O
The dot shows that for every mole of NiSO4 in the crystal, there are 7 moles of H2O.
The ratio of moles of H2O to moles of compound can be determined experimentally in most cases by
heating to remove the water. The compound with the water removed is called anhydrous. In this activity you
will determine the formula for a hydrate of copper (II) sulfate. The formula is determined by comparing the
mass of the hydrated and anhydrous forms of the compound.
Objectives
In this activity, you will:
 Heat a specific amount of the hydrate to remove the water
 Find the experimental and theoretical percentages of water in the hydrate
 Predict a formula for your hydrate
Equipment & Materials
Goggles
Evaporating dish
Hydrated crystals
Forceps
Balance
hot plate
Procedure
1. Clean an evaporating dish with soap and water.
2. Heat the dish on the hot plate to dry.
3. Measure the mass of the cool evaporating dish (record the mass of the evaporating dish in the data table)
4. Add approximately 3 grams of the CuSO4 crystal to the evaporating dish (record the mass in the data table)
5. Heat the hydrated CuSO4 on the hot plate at approximately 215oF (105oC) for approximately 10 min.
6. Remove from the hot plate and let cool
7. Mass the evaporating dish and hydrate (record mass in the data table)
8. After massing, heat dish and crystals again for 2 min
9. Let cool and mass again*
*If the first mass and the second mass are within 0.02 g of each other you are done. If the masses are not within
that range, continue to heat until you obtain two readings that are within that range
1
Data & Analysis
Need to be
within 0.02g
Mass of clean, dry evaporating dish
Mass of hydrate
Mass dish and anhydrous hydrate (after first heating)
Mass dish and anhydrous hydrate (after second heating)
Mass of anhydrous hydrate
Mass of water in hydrate
Calculations
1. Determine the moles of anhydrous hydrate.
(mass of anhydrous hydrate from data table/molar mass of CuSO4)
2. Determine the moles of water in the hydrate.
(mass of water from data table/molar mass of water)
3. Write the experimental formula for your hydrate (look at the formula in the background information).
(determine the mole ratio: divide answers from #1 and #2 by the smallest)
4. Determine the percentage of water in your hydrate using the formula you calculated in question #3.
(molar mass of water * coefficient )/ (molar mass of compound + (molar mass of water * coefficient))
5. Obtain the correct formula for your hydrate from your teacher. Determine the percentage of water in your
hydrate using this formula. (same calculation as #4)
2
6. Compare the experimental (#4) and theoretical (#5) percentages of water in the hydrate.
((\theoretical – experimental\) / theoretical)
Conclusions
1. The method used in this activity to find the percentage of water in the hydrated crystals is not suitable for all
hydrates. Give at least two reasons why this may be so.
2. Why is it necessary to let the evaporating dish cool before massing?
3. Why should the evaporating dish be massed immediately after it cools and not later? How could this have
affected your results?
3