DETERMING THE FORMULA OF A HYDRATE LAB
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Partner________________________________________
Background:
Hydrates are ionic compounds (salts) that take up specific
amounts of water molecules as part of their crystal structure. This
water can be driven off by the application of heat. Since the ratio of
the ionic compound to water molecules is always the same, the
percentage of water should be the same for each sample.
Table salt usually contains a small amount of hydrated
calcium chloride, CaCl2 • H2O, and a hydrated magnesium chloride, MgCl2•6H2O. On humid days these two
impurities absorb moisture and cause the table salt to become wet and to “cake.” To prevent this, some
manufacturers add an anti-caking agent such as magnesium silicate, Mg2Si3O8, which forms a hydrate that
does not become wet.
Purpose:
In this lab you will calculate the percent composition of water in a hydrate and determine the
empirical formula of the hydrate you are working with.
Pre-Lab Questions:
1. What two things make up hydrates?
2. In order to determine the percent composition and the empirical formula of a hydrate, you must know
how much water is in the hydrate. How can you determine this? Hint: Re-read the background
information!
Safety:
Use goggles at all times. Wash your hands thoroughly after handling chemical supplies!
Materials:
Bunsen burner, ring stand, iron ring, clay triangle, crucible with lid, crucible tongs, Nickel
(II) Sulfate (this is the hydrate), electronic balance
Procedure:
1. Being careful not to touch it with your fingers, mass a dry crucible. Record this mass in the data
table.
2. Using a scoopula, measure approximately 2g of Nickel (II) Sulfate into the crucible and record the
exact mass in the data table. Record the initial color of the nickel sulfate here:
3. Turn on the Bunsen burner and place the crucible on the iron ring with the lid on.
4. Observe the contents of the crucible after three minutes and record your observations here:
5. Remove the lid and continue to heat the Nickel (II) Sulfate for about 10 minutes or until it appears
completely light yellow. Now you know all the water has been driven off (evaporated) from the salt.
6. Allow the crucible and compound to cool for 10 minutes. NEVER put a hot dish onto the balance!
7. Mass and record the cooled crucible.
8. Repeat Steps 5-7 until the mass of evaporated water remains constant. Record this data in the
remaining spaces in the Data Table (all the columns may not be used). The first three rows in the
Data Table will be the same for every column.
Data Table
Mass of crucible (g)
Mass of crucible + nickel (II)
sulfate hydrate (g) before heating
Initial mass of nickel (II) sulfate
hydrate (g) before heating
Mass of cooled crucible +
Anhydrous nickel (II) sulfate (g)
after heating
Final Mass of the anhydrous
nickel (II) sulfate (g)
Mass of water that evaporated
from the original hydrate (g)
Post Lab Questions:
You must show all your work for full credit.
1. How many grams of the compound (i.e. the anhydrous salt) were in the hydrate?
2. How many moles of the compound were in the hydrate?
3. How many grams of water were in the hydrate?
4. How many moles of water were in the hydrate?
5. What percent of water was in the hydrate? Hint: % water = (mass water/mass of entire hydrate) x 100
6. Based on YOUR data, what is the empirical formula for your hydrate (NiSO4 • nH2O)? In other
words, solve for n by using your mole ratio.
7. Name the original hydrate based on the chemical formula from Question 5.
8. The known formula for the hydrate is NiSO4 • 6H2O. What is the name of this hydrate? Is it the
same as Question 6?
9. Based on the chemical formula from Question 7, calculate the actual percentage (by mass) of water in
the hydrate.
10. Write the formula for percent error in terms of actual and experimental values.
11. Based on the actual (Question 8) and experimental (Data Table) percentages of water, what was your
percent error for this experiment?
12. Why was it necessary to continue heating the hydrate until the mass remains constant? Explain.
13. List all possible sources of error from your experiment.
14. A 5.0g sample of Cu(NO3)2 • nH2O is heated. After the water has been driven off, 3.9g of Cu(NO3)2
remain. What is the empirical formula of this hydrate?