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ANALYSIS OF HYDRATES
INTRODUCTION
An ionic compound is made of positive and negative ions, called cations and anions,
respectively. At room temperature, all ionic compounds are solid. Within a solid crystal, the ions
are arranged so that a cation is surrounded on all sides by anions, and, an anion is surrounded on all
sides by cations. This pattern of ions is often called a crystal lattice since the pattern looks like a
lattice.
An ionic crystal lattice
A lattice fence
The cations and anions have a spherical shape. Their sizes may vary. For example, in NaCl, the Cl–
ions have a radius nearly twice that of Na+. The radius of Na+ is 95 pm (picometers), while that of
Cl– is 181 pm. When spherical objects are arranged in a pattern, there will be small spaces or holes
between them. Imagine organizing 10 basketballs and 10 soccer balls in an alternating pattern inside
a big box. There would be space in between the balls. In some ionic compounds, H2O molecules can
lodge themselves in these holes.
O─H
H
A water molecule in a crystal lattice hole
Water is an example of a polar compound. A polar compound is sometimes called a dipole. Since
the oxygen atom of water has a partial negative charge, it is attracted to a cation (opposites attract).
The partially positive hydrogens are attracted to anions. These attractions between the water dipole
and the ions are referred to as ion-dipole attractions.
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An ionic compound which has water molecules lodged in the holes between its ions is called a
hydrate. Hydrates are solid at room temperature. The ratio of H2O molecules to ions tends to be
constant for a particular compound. For example, calcium sulfate is regularly found in the form
which has two H2O for every CaSO4 formula unit. This is shown in a formula as CaSO4·2H2O. The
dot separates the two components of this crystal: the ionic portion and the water.
To name this hydrate, the name of the ionic portion before the dot is written as usual (calcium
sulfate), then the word "hydrate" is added with the appropriate prefix to indicate the number of water
molecules (mono=1, di=2, tri=3, tetra=4, penta=5, hexa=6, hepta=7, octa=8, nona=9, deca=10).
Therefore, CaSO4·2H2O is calcium sulfate dihydrate. CoCl2·6H2O is cobalt(II) chloride hexahydrate,
and, Na2CO3·10H2O is sodium carbonate decahydrate.
The formula of a hydrate also has mole implications. Instead of considering individual ions and
molecules, a group such as the mole can be used. In the case of CaSO4·2H2O, this view means that
there are 2 moles of H2O molecules in the lattice for every 1 mole of CaSO4 units.
Many hydrates can have their water driven out simply by heating. The heat breaks the ion-dipole
attraction and the water leaves as vapor. The equation
∆
CaSO4·2H2O(s)
→ CaSO4(s)
+ 2 H2O(g)
represents this process for calcium sulfate dihydrate. The ionic product is called the anhydrous form
of the compound to distinguish it from the hydrate. "Anhydrous" literally means without water. In
some cases the hydrate form and the anhydrous form have different colors. For example,
CoCl2·6H2O is pink and CoCl2 is blue. Different colors usually result only if a transition metal ion is
part of the compound; in this example, Co2+ is such an ion.
PURPOSE
In this lab, you will heat a hydrate, AB·xH2O, to drive off its water. Then you will determine the
ratio of water to ionic units. The identity of AB will be given. Your goal is to find the value of x for
your hydrate.
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Procedure
Safety Notes
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You will be working with flame and hot objects in this lab.
Long hair and loose clothing should be tied back and secured.
When not in use, burners should be turned off.
Do not touch any heated or hot object with your bare hands.
Hot objects should not be placed in a position where they might be bumped.
1.
The hydrate will be heated in a crucible. Before using the crucible, it needs to be fire-cleaned.
Set up a ring stand, with an attached iron ring and a clay triangle on top. Using crucible tongs
to handle the crucible, carefully place it in the triangle. Heat it with a burner until it glows red
hot for about two minutes. (Be sure your flame is blue, not yellow. If it is not blue, ask your
instructor for assistance). Let it cool at least 5-7 minutes.
2.
Using crucible tongs, place your cooled crucible on a balance and weigh. Use the same
balance for all weighings in this experiment. Record the weight.
3.
Add about one gram of your assigned hydrate. Record the combined mass of the crucible and
hydrate to 0.0001 g.
4.
Using crucible tongs, place the crucible and hydrate on the clay triangle. Apply heat, gently at
first, then more vigorous after several minutes. Heat for a total of 15 minutes.
5.
Transfer the crucible to a desiccator, a container which contains a substance that absorbs
moisture from the air. This prevents water vapor in the air from getting back into the crystal
lattice of your compound.
6.
Allow the crucible and contents to cool at least 5-7 minutes.
7.
Place the crucible and contents on the balance, weigh and record.
8.
Return the crucible and contents to clay triangle. Heat vigorously for 5-10 minutes.
9.
Transfer the crucible to a desiccator, and allow the crucible and contents to cool at least 5-7
minutes.
10.
Place the crucible and contents on the balance, weigh and record. Compare this mass with
that taken in step 7. If they differ by no more than 0.005 gram, go on to step 12.
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11.
Repeat steps 8-10 as many times as necessary until you get two readings in a row which differ
by no more than 0.005 gram.
12.
Empty the contents of your crucible into the designated container for wastes. Clean the
crucible as instructed.
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ANALYSIS OF A HYDRATE
Data and Calculations
Name ______________________________ Partner ____________________________
Data
Formula of compound used
· x H2O
Mass of empty crucible
__________ g
Mass of crucible + hydrate
__________ g
Mass of crucible + contents
after first heating
__________ g
after second heating
__________ g
after third heating*
__________ g
after fourth heating*
__________ g
after fifth heating*
*if necessary
__________ g
Calculations
Show your work clearly for each of the following on a separate piece of paper.
Mass of hydrate form of compound
__________ g
Mass of anhydrous form
__________ g
Mass of water driven off
__________ g
Moles of anhydrous form
__________ mol
Moles of water
__________ mol
Ratio of water:anhydrous form
__________
Formula of hydrate
__________
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ANALYSIS OF A HYDRATE
Pre-lab Assignment
Name ______________________________ Lab Section ________________________
1.
Define hydrate.
2.
Name each of the following hydrates.
3.
A.
CaBr2·2H2O ____________________________________
B.
FeCl3·6H2O
C.
MgSO4·7H2O ____________________________________
____________________________________
Complete the following equations and balance.
∆
4.
→
A.
Na2CO3·10H2O
B.
∆
MgSO4·7H2O →
Consider the following data from a procedure performed as outlined in this experiment. A
hydrate of aluminum chloride, AlCl3·xH2O was used.
Empty crucible
Crucible + hydrate
Crucible + contents
after first heating
after second heating
15.807 g
17.061 g
16.503 g
16.499 g
A.
What was the mass of the anhydrous salt? How many moles of anhydrous salt is this?
B.
What was the mass of the water driven off? How many moles of water is this?
C.
Find the mole ratio of water:anhydrous form.
D.
What is the formula of this hydrate?