Lab: Molecular Formula of a Hydrated Salt
Objective: Chemical compounds that contain discrete water molecules as part of their crystalline
structure are called hydrates. Hydrates occur quite commonly among chemical substances since, more
often than not, chemical compounds are prepared in or are recrystallized from aqueous solutions.
Hydrates exist for ionic compounds most commonly, but hydrates of polar and non-polar covalent
molecules are also known. In this experiment you will determine the molecular formula of a hydrated
crystal.
Background: A hydrated crystal is an ionic compound that incorporates water into its crystal structure.
Usually the water of crystallization can be observed as steam escaping from the crystals as they are
heated, or as a condensate on a cooler portion of the vessel used for heating. When the water is
removed through heating, the crystal structure changes. This results in mass changes and sometimes
changes in the appearance of the salt. The amount of water removed from the hydrated salt can be
determined through differences in mass of your sample before and after heating. These mass
differences can be used to determine the formula of the hydrated sample. The formula of a hydrated
crystal is written with the molecules of water following each compound formula (ex. CuSO4•5H2O).
Materials: small Crucible, hydrated Magnesium Sulfate salt, scoop, mass scales, ring stand, ring,
Bunsen burner, crucible tongs, clay triangle.
Procedure:
1. Set up the ring stand, ring, crucible and Bunsen
burner as shown to the right.
2. Turn on the Bunsen burner and heat the empty
crucible for 2 minutes to remove any water.
During this time, discuss your pre-lab questions
with your group.
3. After 2 minutes, remove the crucible from the
heat with the tongs and allow it to cool on the
table top. ***DO NOT PUT HOT CRUCIBLE
ON TOP OF ANYTHING FLAMMABLE***
You cannot touch the crucible with your fingers
for the rest of the lab so you don’t affect the
mass of it. Finish checking your pre-lab
questions and start pre-heating your second
crucible while waiting.
4. After the crucible is cool, weigh it on the mass
scale and record its mass in the data table.
5. Use the scoop to add approximately 2 grams of the hydrated magnesium sulfate salt into your
crucible and record the mass of the crucible with the hydrated salt.
6. Put the crucible with your salt back onto the Bunsen burner for 10 minutes.
7. Remove the crucible from the heat and let your crucible cool on the table top, start your 10
minutes for trial 2 during this time.
8. Mass the crucible with your dehydrated salt once it is cool and record the mass.
9. Re-heat the crucible and salt for an additional 3 minutes, let it cool and re-check the mass. If
the mass doesn’t match the mass of your previous weighing within 0.02 grams, reheat for an
additional 3 minutes, then mass again. Finish your second trial in the same way.
Pre-lab Questions:
1. Calculate the percent by mass of each element in Sodium hydrogen carbonate (NaHCO3) using
the periodic table atomic masses of the different elements and using the equation below.
SHOW YOUR WORK.
2. Determine how much mass would be lost in grams from a 2.55 gram hydrated sample of
CaCl2•4H2O if it was heated and all of the water in the crystal evaporated. SHOW YOUR
WORK.
3. Calculate how many H2O molecules are found in one compound of FeCl3•___H2O if the
hydrated form has a mass of 270.26 grams on the periodic table and the dehydrated form has a
mass of 162.20 grams on the periodic table. SHOW YOUR WORK.
Data Table:
Trial 1
Trial 2

Mass of Crucible (after heating)
____________
__________

Mass of Crucible + Hydrated Magnesium Sulfate
_____________
__________

Mass of Hydrated Magnesium Sulfate
_____________
__________

Mass of Crucible + Dehydrated Magnesium Sulfate
(after 13 – 16 minutes of heating)
_____________
__________

Mass of Dehydrated Magnesium Sulfate
_____________
__________

Mass of Water in Hydrated Salt
_____________
__________

Percentage of Salt in Hydrated Salt
_____________
__________

Percentage of Water in Hydrated Salt
_____________
__________
Formula of Hydrated Salt ___________________
(Use rounded whole number for # of water)
Post-Lab Questions:
1. What error in your calculation of the H2O in your hydrated MgSO4 (higher or lower) would
occur if you had not heated your salt long enough? Explain why.
2. What error in your calculation (higher or lower) would occur if some of your solid had spilled
out of your crucible while you were heating it? Explain why.
Analysis:
 Calculate the formula of your hydrated salt based on the average masses of your two trials.
SHOW YOUR WORK.
 Calculate the percent error of your number of waters (average non-rounded value) in your
hydrate based on the accepted value (7 H2O). SHOW YOUR WORK.
Conclusion:
 Restate the objective of your lab very briefly (1-2 sentences)
 Restate the procedure of your lab very briefly (1-2 sentences)
 Restate the major results of your lab (# waters) and how close they were to the accepted value
(based on % error)