PERCENT COMPOSTION
KNOW THIS: Determining the water of hydration is one example of determining the per cent
composition of a substance. The two processes are related closely.
THEORY: As some crystals form, water gets trapped inside the crystal. Crystals with trapped water are
called HYDRATES. It is interesting to note that the water trapped inside is in a specific
proportion to the rest of the compound. In the case of the compound CuSO4•5 H2O(s)
the compound to water ratio is 1:5. In the case of MgSO4•7 H2O(s), the ratio is 1:7.
The compound without the water is called the anhydrous salt. When you heat the hydrate, you can force out
the trapped water. All that is left is the anhydrous salt. By knowing the starting mass and the ending mass you
can determine the percent composition of water.
trapped water
(SO4)22+
Cu
trapped water
(SO4)2-
Cu2+
(
Cu2+
2+
Cu
(SO4)
(SO4)22-
Cu2+
trapped
water
(SO4)2-
Hydrate
Cu2+
Anhydrous Salt
Crystals with trapped water
Dried crystals
In the case of CuSO4•5 H2O(s) the hydrate is blue, due to the presence of some copper ion dissolved in the
trapped water. When the water is driven off, the anhydrous salt turns whitish in color.
energy
CuSO4 • 5 H2O(s)
 
blue
crystals
 CuSO4(s) + 5 H2O(g)

the water is first a liquid and then is vaporized away
white
powder
How Do I Calculate the Percent Composition?
In this experiment, you are to determine what percent of the hydrate’s mass is due to water’s mass.
Now, percent calculations are pretty straight-forward. As a general rule when performing a per cent
calculation you can follow the generalized formula found on Table T of the NYS Reference Charts.
% = Part x 100
Whole
or as I might say:
% = Part I'm looking for x 100
Total possibility
For example, your gross pay is $135.00. Your paycheck reflects a $20.00 dollar tax, thus, your take-home
(net) pay is only $115.00. What percent of your gross pay is retained in taxes?
percent = Taxes retained
Gross Pay
percent = 20.00 x 100
135
x 100
=
per cent in taxes = 14.8 %
The percent by mass of the water of hydration can be determined experimentally according to an
application of the above equation:
% H2O
=
the mass of water driven off
x 100
the total mass of the hydrate
What is meant by the term "Heating to a Constant Mass"?
In order to ensure all of the water is gone, you must heat and mass the sample multiple times.
It is reasonable to suggest that a single heating is insufficient to remove all of the water. One means of
ascertaining that all water is removed is by "heating to a constant mass". This process involves multiple
heatings. After your first heating/cooling/ massing period, you should heat the same sample again - for
about two minutes. Cool and re-mass. When the mass changes by only 0.001g - you're
probably at a constant mass. When the mass changes by more than 0.001g - repeat the two minute
heating/cooling/re-massing - until the mass no longer changes.
This is called heating to a constant mass. When all of the water is removed, the mass will no longer
change.
How can I calculate the per cent error?
Per cent error reflects the absolute value of the variation between a piece of experimental
datum and a theoretically accepted piece of datum. The equation is:
per cent error
= measured value - accepted value x 100
accepted value
Example: A student found that 38.80 mL of CO2(g) were produced during an experiment.
Theoretically 41.30 mL of CO2(g) should have been produced. Calculate the percent error.
Equation :
per cent error = measured value - accepted value x 100
accepted value
Substitution :
per cent error =
-2.50 mL x 100
41.30mL
= 6.053268765 = 6.05 %
Answer : 6.05%
A positive % indicates that the measured value was greater than the accepted value.
A negative % indicates that the measured value was less than the accepted value.
PROCEDURE :
1. Put on your safety goggles
2. Obtain a porcelain crucible. Handle the crucible with crucible tongs as you were taught.
3. Set up a heating system consisting of: a ring stand, ring clamp, clay triangle and Meeker
burner, as demonstrated by your instructor. You what to gently heat the contents. So,
adjust the ring clamp so that the bottom of the crucible is a good three inches away from
those inner blue flames. For this lab we shall try a gentle, but longer heating period.
Now, light the Meeker burner and quickly, adjust the ring clamp, clay triangle and crucible
to be at the appropriate height to ensure a gentle heating. Extinguish the flame.
4. Gently wipe out the crucible, using a DRY paper towel. NEVER place the crucible in
water. Do not wipe out the crucible with a wet towel!! The porcelain absorbs water.
5. Using an electronic balance, mass the crucible. Record this mass to 0.001 gram
6. Tare the balance and mass about 2.000 grams of (MgSO4•7 H2O) crystals into the crucible.
Record the mass to 0.001 g.
7. Using crucible tongs, place the crucible containing the hydrate on the clay triangle.
8. Light the Meeker burner, & gently heat crucible and contents for about 8 minutes. If you
are quick, you might listen for and/or see water as it sizzles out of the crystals.
CAUTION! Heating too rapidly at this point can cause the contents to splatter out
9. Cool the crucible and its contents. Let it sit on the porcelain plate. When you're unsure if it's
cool, call over your instructor. Cooling takes a good 5 minutes.
10. Mass the crucible and the anhydrous salt. Record the mass to 0.001 g
11. Repeat steps 8, 9 and 10 until a constant mass is achieved and recorded. For the second
heating, ( &/or until the constant mass is reached) heat for only 4 minutes, and then allow the
crucible to cool for another 5 minutes or so. Mass the crucible and the salt again.
Continue the re-heating until a constant mass is achieved.
Because you need to achieve a constant mass, a 3rd , 4th, or even 5th heating (each at 2
minute intervals at high heat) may be required.
For our work a constant mass is achieved when there is a maximum change in the
mass of the crucible contents or 0.001 grams (or 0.000 grams) between heating sessions.
13. When finished, clean your crucible by removing any residue of the anhydrous salt that
remains. Put the anhydrous salt into the waste basket or rinse it down the drain with water.
DO NOT RINSE THE CRUCIBLE IN WATER. NEVER ADD WATER, AT ANY
POINT TO THE CRUCIBLE.
NAME _______________________________________
WATER OF HYDRATION
TITLE : DETERMINATION OF THE PERCENT WATER OF HYDRATION
OBJECTIVES :
a) To determine the percentage by mass of water in MgSO4•7 H2O
b) To learn the terms HYDRATE (Salt) and ANHYDROUS SALT
DATA:
Data for The Water of Hydration for MgSO4• 7 H2O
1. Mass of empty crucible
2. Mass of the hydrate (salt) (just the sparkling crystalline material)
3. Mass of crucible and hydrate (salt)
1st heating
2nd heating
3rd heating
(if necessary)
4. Mass of crucible & the anhydrous salt
5. Mass of the just the anhydrous salt
6. Mass of H2O driven off
(dried out powder)
(water lost or released by the hydrate, due to heating)
*7. per cent water of hydration (measured value from lab)
8. Theoretical value of the % water of hydration for
MgSO4•7 H2O
*9 Percent Error
51.2 %
%
CALCULATIONS:
*7. Calculate the percent water of hydration of your compound. This is your measured or experimental value.
Table T has the per cent composition equation
Equation from Table T (or theory sheet):
Substitution of measurements:
Answer __________________________
Turn this over to learn about calculating % error
*9 The accepted value for the per cent water of hydration of MgSO4•7 H2O(s) is 51.2%. Use your
experimental value (data table line #7) to determine the per cent error. A positive quotient means your value is too high
(above the accepted value), while a negative quotient means that your answer is less than the accepted value.
Equation:
% error =
measured value – accepted value x 100
accepted value
Substitution:
Answer: ________________% (sig figs????)
a) Is the experimental value higher than the accepted value or below the accepted value _________
QUESTIONS:
1) Explain how your experimental value for the percent water of hydration would be DIFFERENT,
had you not heated the hydrate to a constant mass. Be sure to defend your reasoning.
2) United States pennies minted after 1982 are a zinc metal disc, wrapped in a thin copper envelope. On
average, a post-1982 penny masses at 2.5 grams. The copper envelope is surprisingly thin, but tough. It resists
the attack of most acids. The inner zinc core is vulnerable to the attack of acids. If you etch into the rim of a
penny with a steel file, you can reveal the inner zinc core.
A student knows that if she nicks open a post 1982 penny, she can reveal some of the inner zinc core.
She does so, and then carefully places the penny into 50.0 mL of hydrochloric acid. The zinc metal
chemically reacts with the acid overnight. The copper envelope does not.
a) She wishes to determine the per cent composition of a post 1982 penny. DESIGN a data table she
SHOULD use to collect her data. Be sure to include all necessary data points to be collected. You
do NOT need to include actual data however….just the design of the table.
b) The current composition of a US penny is 97.5 % zinc and 2.5 % copper. Calculate the average
number of grams of each metal in an average penny.
CONCLUSION: (Refer to only the first objective of the theory reading. Re-state the objective, declare
your percent composition (experimental), declare the accepted value, declare how your result varies from
an accepted value and declare sources of possible error)
REFLECTION: Complete your reflection using the following stems:
I was surprised that...
I am really piecing past courses with this work because...
Were I able to do it again I would like to...
I wish I knew more about...
I don’t fully understand...
I thought it was really interesting when...
I have made a connection between...
The results were best when…
I learned ...
I never knew that...
I don’t agree with...
The question I still have is…
This lab was different because...
This lab was like all the others because...
Aside from re-doing the lab, I think the
next question (or next best step) to be...