CHEM115 LAB 4 HYDRATED CRYSTALS
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Brianna Krzyminski
CHEM115 Lab 4
TITLE:
Hydrated Crystals
BACKGROUND INFORMATION:
Many compounds are formed in reactions that
take place in water solutions. The water is then evaporated to obtain the crystalline
compound. In some cases, the water molecules are weakly attracted to the ions or
molecules that make up the compound and are retained within the crystal structure.
Crystalline compounds that retain water during evaporation are referred to as being
hydrated
or are said to contain
water of hydration
. The ratio of moles of water to
moles of compound is a small whole number. For example, the formula for the
hydrated compound copper(II)sulfate is:
CuSO
4
•
7H
2
O
The formula shows that for every mole of CuSO
4
in the crystal, there are 7 moles of
H
2
O. The ratio of moles of H
2
O to moles of compound can be determined
experimentally in most cases by heating to remove water. The compound with the
water removed is called
anhydrous
.
OBJECTIVES:
In this experiment, you will
*Heat a specific amount of hydrated magnesium sulfate to remove the water.
*Find the experimental and theoretical percentages of water in the hydrate.
*Predict the empirical formula for the hydrated magnesium sulfate
EQUIPMENT:
iron ring
wire gauze
Bunsen burner
crucible tongs
evaporating dish
watch glass
ring stand
desiccator
PROCEDURE:
1.)
Prepare a data table as directed in the analysis.
2.)
Make sure that your evaporating dish is clean and dry.
3.)
For the remainder of he lab, avoid handling the evaporating dish with your
hands. Use either the crucible tongs or paper towel.
4.)
Measure the mass of the evaporating dish.
5.)
CAUTION:
MgSO
4
are irritating, avoid breathing the fumes. Add about 3 grams
of the MgSO
4
crystals to the evaporating dish. The mass does not have to be
exactly three grams but record exactly how much of the crystals that you use.
6.)
Place he evaporating dish and the crystals on the gauze pad and iron ring stand.
7.)
Heat the evaporating dish and crystals with a medium flame for three minutes.
8.)
Stop heating the evaporating dish. When the dish is cool enough to remove,
immediately mass it.
10.) Clean evaporating dish with water.
ANALYSIS:
1.)
Prepare a table for your data, using the table below as a guide.
DATA TABLE:
Before Heating
(Hydrated MgSO
4
)
After Heating
(Anhydrous MgSO
4
)
Evaporating Dish + MgSO
4
Mass
53.1424 g
51.8455 g
Evaporating Dish Mass
50.0815 g
50.0815 g
MgSO
4
Mass
3.0609 g
1.7640 g
CALCULATIONS TABLE:
Mass of H
2
O in Hydrate MgSO
4
1.2969 g
Moles Anhydrous MgSO
4
0.015 mol
Moles H
2
O in Hydrate MgSO
4
0.072 mol
2.)
Show your calculations for each of the following:
a.)
Determine the number of moles of anhydrous magnesium sulfate.
1.7649 g MgSO
4
1 mol MgSO
4
0.015 mol MgSO
4
120.0 g MgSO
4
b.)
Determine the number of moles of water in the hydrate.
1.2969 g H
2
O
1 mol H
2
O
0.072 mol H
2
O
18.0 g H
2
O
c.)
Using your data, determine the percentage of water in the hydrated
crystals of magnesium sulfate.
1.2969 g H
2
O
100
42.37% H
2
O
3.0609 g Hydrated MgSO
4
d.)
Assume that the correct formula for hydrated magnesium sulfate is
MgSO
4
7H
2
O. Using this formula, calculate the theoretical percentage of
water in the hydrated magnesium sulfate.
MgSO
4
= 49%
7H
2
O = 51%
Mg
x 1 x 24
= 24 g
H
x 2 x 1
= 2 g
S
x 1 x 32
= 32 g
O
x 1 x 16
= 16 g
O
x 4 x 16
= 64 g
= 18 g x 7 =
126 g
=
120 g
120 g / 246 g =
0.48780488
126 g / 246 g =
0.51219512
e.)
Compare the experimental percentage of water in the hydrate with the
theoretical percentage by calculating your percent error.
|
51.0 – 42.37
|
100
16.92 %
51.0
QUESTIONS:
1.)
From your data table, compare the number of moles of anhydrous MgSO
4
to
the number of moles of water in the hydrate, this is our ratio. Using the
ratio that you obtained from your data table, write the formula for
hydrated MgSO
4
determined from the experiment (round your numbers to
the nearest whole number when writing the formula).
0.015 mol MgSO
4
: 0.072 mol H
2
O
0.015
1 MgSO
4
0.015
5 H
2
0 : 1 MgSO
4
2.)
The method used in this experiment (heating) to find percentage of water
in the hydrated crystals are not recommended for all hydrates. Give at
least two reasons why this may be so.
The reason heating is not suitable for obtaining water content of all hydrates is
that not all hydrates can survive hot temperatures required to remove water.
Another reason heating may not be useful is some hydrates are either very
unstable or reactive in their anhydrous form and will react or decompose
without the water present.
3.)
a.) Why is it necessary to let the evaporating dish cool before measuring it?
Weighing a hot dish will transfer the heat through the metal pan of the
balance and ruin the electronic balances circuitry. Additionally, weighing hot
objects gives inaccurate results because of convection currents, as hot air
around the object rises, the mass can be made to look artificially low.
b.)
Why should the mass of the evaporating dish be measured immediately
after it cools, and not later?
The longer the MgSO
4
is away from the heating source, the more likely it will
begin to loosely reattach to H
2
O molecules in the atmosphere, rehydrating
the MgSO
4
thereby affecting the accuracy of the experiment results.
4.)
What is a desiccator? How does a desiccator work?
Desiccators are sealable /airtight enclosures which contain desiccants to ensure
a moisture free internal environment. They work by ensuring an internal
atmosphere that is free of H
2
O molecules.
5.)
How could your experimental results be affected if you did not use a
desiccator when cooling the evaporating dish and its contents?
A desiccator was not used during the cooling process for our experiment. This
could have affected the accuracy of our results because during the 10-minute
cooling period the MgSO
4
was able to possibly begin rehydrating by reabsorbing
H
2
O molecules that naturally occur in the environment.
0.072
4.8 H
2
O
0.015
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