Water in Crystals
Experiment #9
Introduction
In aqueous solution, water molecules are attached
to dissolved ions. If we could actually see
something so small, the hydrated (surrounded by
water) ion would probably look like a clump of
water molecules with a charge. The ion would be
in the center of the clump, and only the water
molecules on the outside would be visible from
the outside.
If the solution is left undisturbed so that the water
may evaporate, the ions join together to form
crystals. In some substances, a few water
molecules remain attached to each ion and
become a part of the crystal structure. The crystal
looks, and in fact is, quite dry and solid. The
water molecules do not form a liquid because they
are kept apart by the crystal structure and by their
attraction for the ions. These crystals are called
hydrates.
Some examples of hydrates:
CaSO4·2H2O calcium sulfate dihydrate
CoCl2·6H2O cobalt(II) chloride hexahydrate
In a hydrate the water molecules are a distinct part
of the compound but are joined to the salt by
connections that are weaker than the connections
in the salt or the connections in the water
molecules. Notice that a dot is used in the formula
to show the water molecules are connected to the
crystal.
Will any properties of the crystal be changed
when the water is removed? Do all of the crystals
hold the same amount of water?
1.
2.
3.
4.
5.
Prelab Reading and Problem Assignment
Read this lab.
Create a table for your data.
Create a table for your calculations.
Calculate the percent water in each of the
example hydrated crystals listed above.
A 1.42 g sample of hydrated magnesium
sulfate is heated so that all of the water of
hydration has been removed. The anhydrous
crystal has a mass of 0.69 g. Determine the
formula of the hydrate.
106746972
Experimental Question
How much water is contained within a hydrated
crystal?
Experiment Overview
A sample of hydrated copper(II) sulfate is massed
and then heated to constant mass. The mass of the
hydrate and its water are then determined and
analyzed.
Water is added dropwise [drop by drop] to the
anhydrous [without water] salt and observations
are recorded.
Procedure
Obtain a small beaker, write your initials and
period number on it using the provided Sharpie
marker, and mass it. Place a small crystal of
hydrated copper(II) sulfate in the beaker and mass
them together. The instructor will place the beaker
in the drying oven overnight to dry.
After measuring the mass of the beaker and
anhydrous crystal, place the anhydrous crystal in a
small test tube. Use a plastic dropper to add water
dropwise, recording observations of any change,
until 2 or 3 mL of distilled water has been added.
Calculate and report the following:
1. Mass of the hydrate.
2. Mass of the anhydrous crystals.
3. Mass of water in hydrate.
4. Percent water in the hydrate.
5. Formula of the hydrate. The formula is
always written in the form 1 mole anhydrous
salt · x moles H2O.
Data Analysis
The class analyzes the percent water data using
mean, standard deviation, 95% range, % error,
and % sigma.
Comment on the accuracy and precision of the
class data.
Calculate the percent error in your percent water
value using the correct value provided in class.
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