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Empirical Formula of a Hydrate
The strong dipole of water, which causes it to be an excellent solvent for ionic
substances, also causes water molecules to attach themselves to ions in solution. Such
ions are called hydrated ions. When some solutions of hydrated ions are evaporated, the
water molecules are so strongly attracted to the ions in solution that they remain attached
as crystallization occurs. Water molecules are incorporated into the crystal structure.
This water is called water of hydration.
Crystals that have formed in this way appear to be perfectly dry, yet when heated
yield large quantities of water. The crystals change form, sometimes, even color, as the
water is driven off. This indicates that the water was present as an integral part of the
crystal structure. Such compounds are called hydrates. Usually, the amount of water
present in a hydrate is in a whole number molar ratio to the moles of the anhydrous salt,
which is the ionic compound without water. An example of a hydrate is magnesium
sulfate. Its formula is MgSO4*7H2O, indicating that seven moles of water are combined
with one mole of magnesium sulfate in the crystalline form.
PURPOSE:
In this experiment you will be given an unknown hydrate. You will find the mass
of water driven off by heating and the amount of anhydrous salt that remains. From this
information, the molar ratio of water to anhydrous salt will be calculated. Finally, the
identity of the hydrate will be determined.
PROCEDURE:
1. Place a clean, dry evaporating dish and watch glass in a pipe-stem triangle
mounted on an iron ring stand. Heat with a nonluminous flame for two or three
minutes.
2. Let the evaporating dish cool for at least 3 minutes. When the evaporating dish
and watch glass are cool enough to touch, transfer it to a balance. Find the mass
to the nearest 0.01 g.
3. Obtain a small amount of your unknown hydrate and transfer it to your
evaporating dish. The amount should fit in this square =>
4. Mass and record the evaporating dish and watch glass with the unknown hydrate
crystals to the nearest 0.01g.
5. Place the watch glass over the evaporating dish to prevent any solid material of
“splattering” out, but leave a gap so steam can escape. Heat the crystal gently for
approximately 5 minutes. The solid will pop and splatter and change color.
NOTE: If the edges of the solid appear to be turning brown, reduce the heat
momentarily and then begin heating again at a slower rate.
6. Allow the evaporating dish and watch glass to cool. When the evaporating dish
and watch glass is cool enough to touch, transfer it to the balance and find the
mass.
7. Use a wood splint to break up any chunks of material stuck together. Heat the
evaporating dish and watch glass again for 5 minutes. Cool and find the mass.
Repeat this step until the mass does not change.
8. What is now in your evaporating dish is an anhydrous salt, or the ionic compound
without water.
DATA AND OBSERVATTIONS:
1) Mass of evaporating dish and watch glass
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2) Mass of evaporating dish, watch glass, and hydrate
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3) Final mass of evaporating dish, watch glass, and anhydrous salt after all
heatings.
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4) Calculate molar mass of anhydrous salt used, CuSO4
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DATA ANALYSIS
CALCULATIONS AND RESULTS:
1. Calculate the initial mass of hydrate.
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2. Find the mass of water heated off (mass with hydrate - final mass).
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3. Calculate the number of moles of water lost
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4. Calculate the mass of the anhydrous salt.
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5. Determine the number of moles of anhydrous salt.
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6. Determine the mole ratio of the salt to water by taking moles of water lost
divided by moles of anhydrous salt.
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7. Write the empirical formula for the hydrate.
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8. What is the percent composition of the anhydrous salt?