Chemistry 151
Lab 5: Hydrated Compounds
Last Updated: Dec. 2013
Introduction
When ionic compounds form, there are sometimes gaps or cavities within the crystal lattice that are large
enough to trap water molecules, forming a hydrated compound. For example, Epsom salt is hydrated
magnesium sulfate, with seven water molecules per formula unit, MgSO47H2O.
You can often remove the water in a hydrated compound by heating, forming the anhydrous form of the
compound. In many cases the anhydrous and hydrated forms of the compound will have distinctly different
colors. Chromium (III) chloride, for example, has a dark green color when anhydrous, but is violet as
CrCl36H2O.
With many anhydrous compounds, simply absorbing moisture from the air can readily hydrate them. Such
compounds are said to be hygroscopic. In fact, there are some hygroscopic compounds that absorb so
much moisture that a pool of water will form around the solid. These types of compounds are said to be
deliquescent.
In this lab you will examine the properties of several different hydrated compounds. You will also
experimentally determine the chemical formula of a hydrated compound.
Procedure
Part I: Sodium Hydroxide
1. Place a pellet of sodium hydroxide on a watch glass and observe it over the course of the lab.
Part II: Calcium Chloride
1. Place a small amount of CaCl22H2O in a medium-sized test tube.
2. Using a Bunsen burner with a cool (yellow) flame, gently heat the tip of the test tube for no more than
30 seconds. Record your observations.
3. Continue heating for another minute or two, until all of the water has been evaporated (including on
the sides of the tube)
4. Set the test tube aside and see if any other changes occur.
Part III: Cobalt (II) Chloride
Repeat Part II using CoCl26H2O
Part IV: Copper (II) Sulfate
Repeat Part II using hydrated copper (II) sulfate.
Part V: Experimentally Determining the Formula of Hydrated Copper (II) Sulfate
1. Rinse a crucible with 6M nitric acid.
2. Attach an iron ring to a ring stand and set a clay triangle over the ring. Place the crucible in the
triangle and heat it to dryness with a Bunsen burner.
3. Once dry, allow the crucible to cool and measure its mass with a lid.
4. Add about 1 gram of hydrated copper (II) sulfate to the crucible and record the combined mass of the
crucible, lid and solid.
5. Return the crucible to the clay triangle, set the lid aside, and heat the solid for ten minutes.
6. Remove the heat, cover the crucible with the lid and allow it to cool.
7. When cool enough to handle, record the new mass of crucible, lid and solid.
8. Repeat steps 1-7 for a second determination.
Waste Disposal
Solid waste should be disposed of in the waste hood. If you have difficulty removing the solid from the test
tube, dissolve it with water. Nitric acid can go down the drain with running water.
Name: _____________________________
Section: ________
Data
Part I: Sodium Hydroxide
Record your observations in the space below.
Part II: Calcium Chloride
Record your observations in the space below.
Part III: Cobalt (II) Chloride
Record your observations in the space below.
Part IV: Hydrated Copper (II) Sulfate
Record your observations in the space below.
Part V: Experimentally Determining the Formula of Hydrated Copper (II) Sulfate
Trial 1
Trial 2
1) Mass of crucible and lid, g
_________
_________
2) Mass of crucible, lid and
hydrated comp., g
_________
_________
3) Mass of hydrated comp., g
_________
_________
4) Mass of crucible, lid and
anhydrous comp., g
_________
_________
5) Mass of anhydrous comp., g
_________
_________
6) Moles of copper sulfate
_________
_________
7) Mass of water, g
_________
_________
8) Moles of water
_________
_________
9) Mole ratio, water to
copper sulfate
_________
_________
Average
(rounded to nearest integer)
Formula of hydrated compound:
_________
_____________________
Show your work for the following calculations from Trial #1
a) Mass of water
d) Moles of anhydrous compound
b) Mass of anhydrous compound
e) Ratio of water to compound
c) Moles of water
Name: _____________________________
Section: ________
Post-lab Questions
1. Which compound(s) studied today appeared to be hygroscopic? Explain.
2. Which compound(s) studied today appeared to be deliquescent? Explain.
3. A desiccator is a special airtight container used to store moisture-sensitive compounds. Usually the
bottom of the container is lined with either anhydrous calcium chloride or anhydrous cobalt (II) chloride,
which absorbs any residual moisture that’s initially present, or any that’s introduced after you open or close
the desiccator. Once the compound has become fully hydrated, you can either reheat it to return it to its
anhydrous state, or simply replace it with a fresh batch.
Based on your observations in this lab, which compound—calcium chloride or cobalt (II) chloride—would
be the better choice for this application? Explain.
4. How would the following errors affect the calculated mass of water in Part V? Would it make the value
incorrectly high, incorrectly low, or would there be no effect? Explain your answers.
a) A student skipped the first two steps and didn’t clean the crucible with HNO3. There were impurities
present that were removed as the salt was being heated.
b) A student skipped the first two steps and didn’t clean the crucible with HNO 3. There were impurities
present that weren’t removed as the salt was being heated.
c) The salt was weighed on a piece of weighing paper then transferred to the crucible, but some of the solid
was lost during the process. The crucible, lid and salt weren’t reweighed after the transfer to get the
combined mass of the three (line 2 of the data sheet). Rather, the recorded mass of the solid (line 3) was
simply added to the previously recorded mass of the crucible and lid (line 1).
Name: _____________________________
Section: ________
Pre-lab Questions
1. Define the following terms:
a) Hydrated salt
b) Anhydrous salt
c) Hygroscopic
d) Deliquescent
2. In Part V, why is the lid removed from the crucible while heating the solid? Why do you place the lid on
the crucible when letting it cool?
4. A 3.00 g sample of a hydrated calcium carbonate compound, CaCO3xH2O, was heated. Afterwards, the
anhydrous compound weighed 2.21 g.
a) Calculate the mass of water in the hydrated compound.
b) Calculate the moles of water in the hydrated compound.
c) Calculate the moles of calcium carbonate in the hydrated compound.
d) Find the ratio of water to calcium carbonate to determine the value of x.
e) Write the correct formula of the hydrated compound: _______________________
5. Watch the following video on using a Bunsen burner: http://youtu.be/9QpIcX-xw1Q
a) What does adjusting the valve on the bottom do?
b) What does adjusting the barrel of the burner do?