Percent of Water
When many ionic compounds are crystallized form a water solution, they include
individual water molecules as part of their crystalline structure. If the substances are heated, this
water of crystallization may be driven off and leave behind the pure anhydrous form of the
compound.
I.
% of water in a hydrated compound:
1. Mass an empty, clean and dry evaporating dish. Record mass in a data
table.
2. Mass out about 3 grams of hydrated copper II sulfate. Record mass in the data
table.
3. Carefully transfer the copper II sulfate to the evaporating dish, and re-mass.
Record that value in your data table.
4. Heat the evaporating dish with a very low heat until you witness a complete
change. Record your observations in the data table.
5. Allow the dish containing the dehydrated crystal to cool. Once cool, get the
mass, and record that value in your data table. Before throwing out the compound,
drop 1-2 drops of water back on the crystal. Record what happens.
II.
% of water in an unknown compound:
1. Mass an empty, clean and dry evaporating dish. Record the mass in a second
data table.
2. Mass out 1.5 grams of the second hydrated crystal. Record the mass in your
data table.
3. Carefully transfer the second hydrated crystal to the dish, and re-mass. Record
that value in your data table.
4. Heat the dish with a very low heat until you witness a change. Record your
observations in the data table.
5. Allow the dish containing the substance to cool. Once cool, get the mass, and
record that value in your data table. Before throwing out the compound, drop 1-2
drops of water back on the crystal. Record what happens.
III.
Background
Explain what a hydrated crystal is, how to calculate molar mass of a substance, and
how to calculate percent composition for a hydrated crystal. Describe how to
calculate the empirical formula from percentages.
IV.
Calculations
Part A
1. Calculate the amount of water lost from the compound.
2. Using the value you calculated in question 1, calculate the % of water in
the compound.
3. Using the chemical formula found on the label of the bottle for each
compound, compute the theoretical % water in the compound, using the
periodic table.
4. Calculate the percent error, then explain what the potential sources of
error were.
% error = experimental-theoretical x 100
theoretical
Part B
1. Calculate the amount of water lost from the compound.
2. Using the value you calculated in question 1, calculate the % of water in
the compound.
3. Use the mass of the dried compound after heating to calculate the
percentage of the original sample made up of the ionic compound.
4. Use both answers from questions 3 and 4 to find an empirical formula
for the hydrated crystal.
V.
Questions:
1. Re-hydrate the heated crystal from parts 1 and 2. What do you notice?
Which substance would be a good indicator of atmospheric moisture?
Explain.
2. Popcorn pops because water trapped inside the kernel turns into gas and
the pressure causes the skin of the kernel to burst open. How might you
go about trying to find the % water inside of a kernel of popcorn?