WATER OF HYDRATION
EXPERIMENT 17
PURPOSE
To determine the percent water in copper(II) sulfate pentahydrate.
DEFINITIONS
Water of Hydration, conservation of mass.
INTRODUCTION
Many salts, when crystallized from water solutions, take up definite proportions of water as a part of their
crystal structures. This “water of hydration” may be driven off by heating. The Law of Conservation of
Mass can be used to determine the percent of water in a hydrated salt from both experimental data and the
chemical formula for the hydrate.
SAFETY
 Don’t touch copper(II) sulfate pentahydrate directly with your hands. It is toxic. Wash your hands
when the experiment is done.
 Clean your work area when you are finished.
 Put all waste residues in the designated container.
PROCEDURE
1. Clean a small test tube and then warm it in the flame of a Bunsen burner until dry. Let it cool, and then
mass the test tube to the nearest 0.1 g. Record in Data Table 1.
2. Add copper(II) sulfate pentahydrate to the test tube until it is about one-fourth full, and mass the test
tube plus hydrate to the nearest 0.1 g. Record in Data Table 1.
3. Warm the test tube plus hydrate in the burner, slowly at first, to drive off the water of hydration. When it
appears that no more water is being driven off, heat the test tube for another five minutes.
4. Allow the test tube to cool, and then mass the test tube to the nearest 0.1 g. Record in Data Table 1.
5. Place all waste chemical in the designated container. Clean your test tube.
ANALYSIS
2.
3.
4.
5.
6.
7.
8.
1. State the most important safety concern in this lab and the required precaution you took.
Write the chemical formula for copper(II) sulfate pentahydrate and determine its molar mass.
Determine the mass of copper(II) sulfate pentahydrate added to the test tube.
Determine the mass of the water that was lost by heating
Calculate the percent of water in copper(II) sulfate pentahydrate from your experimental data.
Calculate the theoretical percent of water in copper(II) sulfate pentahydrate based on the chemical
formula for this compound.
Calculate percent error in the percent of water in the hydrate using the results from step 5 and step 6.
Honors: What was the most probable source of measurement error that would lead to an incorrect
density determination?
OBSERVATIONS
DATA TABLE 1: MASS MEASUREMENTS
Item
empty test tube
test tube plus copper(II) sulfate pentahydrate (before heating)
test tube plus anhydrous salt (after heating)
Mass