Lab # 2
The Formula Of Hydrates
Hydrates
 Hydrates are ionic compounds that have a specific
number of water molecules attached to them.
 Examples:
 Calcium Chloride dihydrate – CaCl2 * 2 H2O
 Copper (II) Sulfate pentahydrate – CuSO4 * 5 H2O
Analyzing a Hydrate
 When a hydrate is heated the water molecules that are
attached to it evaporate into the air.
 This will result in the sample having less mass than it
did originally.
 This fact can be used to determine how many water
molecules are attached to each formula unit in the
hydrate.
Example
 Suppose we have a 5.00 g sample of BaCl2 * X H2O
 And we want to determine X.
 We would heat up our previously weighed sample until
we are sure that all of the water has been evaporated.
 Then we reweigh our sample and see that it now only
has a mass of 4.26 g.
 This tells us that we lost a total of 0.74 g of water.
 We now know that our original sample was 0.74 g
water and 4.26 g of BaCl2.
 We need to now convert these two numbers into moles.
 4.26 g BaCl2 x (1mol/208.23 g) = 0.0205 mol BaCl2
 0.74 g H2O x (1mol/18.02 g) = 0.041 mol
 We then set up our formula:
 0.0205 BaCl2 * 0.041 H2O
 Dividing by the smaller of the two numbers gives us:
 BaCl2 * 2 H2O
Materials
 Goggles
 Ring stand
 Crucible and lid
 Iron ring
 Clay triangle
 Bunsen burner
 Scale
 Tweezers
 MgSO4 * X H2O
Procedure
 Set up your ring stand, iron ring, and clay triangle.
 Weigh your crucible and lid, record your results in your data
table
 Using weighing paper, weigh out about 3 g of MgSO4 * X
H2O and then put it in the crucible.
 Reweigh your crucible and lid with the sample inside, record
your results in your data table
 Use subtraction to calculate the exact mass of MgSO4 * X
H2O in your crucible, record your results in your data table.
 Put your crucible now containing your sample carefully
onto the iron ring with the lid slightly ajar.
 Call Mr. Berger over to light your Bunsen burner.
 Slowly wave the flame under the crucible to heat it
slowly. We want to heat slowly to ensure that only
water is being driven off. If you hear crackling or see
sparks move the flame away IMMEDIATELY.
 After heating slowly for about 10 minutes turn off the
burner.
 Using the tweezers CAREFULLY remove the crucible
from the clay triangle, set it on the table with the lid
COMPLETELY on. We want the lid on so that no water
from the air can be absorbed by our now dehydrated
sample.
 When your crucible is cool enough to touch, weigh it
again with the lid on.
 Use subtraction to determine the mass of your
dehydrated sample.
Analysis Questions
1. Use the data you collected to calculate the formula for
hydrated MgSO4. Show all work. What would be the
name of this compound?
2. How do the appearances of the hydrated and
dehydrated crystals compare? How are they different?
3. If the hydrate’s formula is MgSO4 * 7 H2O, what is the
percent error in your experiment? What are some
possible sources of error? What procedural changes
could you make to reduce the error?
Percent Error Calculation
 Percent error is calculated by subtraction your
experimentally determined number from the actual number
then dividing by the actual number.
 Example:
 If I do this experiment using MgSO4*7 H2O, and I determine
that the amount of water molecules attached to each MgSO4
is 6.98 my percent error is calculated as:
 [(6.5 – 7)/7] x 100 = - 7.1% (yes its ok to have a negative
percent. That just means that your experimental number
was lower that the actual number)