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Lab # 16: Empirical Formula for a Hydrate
Accelerated Chemistry 1
Problem
What is the percentage of water in a hydrated salt? What is the molecular formula for the hydrate?
Introduction
A hydrate is a compound that contains water in its crystal structure. The water may be removed from
the salt in the laboratory by heating the salt. The salt without the water is called an anhydrous salt.
Here are two examples of hydrates:
CaSO4·2H2O
Calcium sulfate dihyrdate
MgSO4·7H2O
Magnesium sulfate heptahydrate
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 ionic bonds in the salt or the covalent bonds in the water
molecules. Notice we use a dot to connect the water units to the salt formula. Also note the name of
the compound. The salt is named properly as an ionic compound and a prefix is used to denote the
number of water molecules complexed to the salt in a simple crystal of it. The coefficient in front of
water in the formula also represents the mole ratio of the water to the salt. For example, there are 7
moles of water in the compound for 1 mole of magnesium sulfate.
In this experiment you will measure the mass of hydrated copper (II) sulfate, then remove water from
the crystals and measure the mass of the anhydrous salt. The data gathered will allow you to determine
the percent water in the hydrated salt and also the empirical formula for the hydrated salt.
Pre-lab Assignment
Read the Introduction and Procedure. Answer the following questions:
1. What information is needed to determine the percentage of water in your hydrated sample?
2. A hydrate has the formula MgSO4· 7H2O. What is the percent water in this hydrate?
Safety
Hot items look the same as cool items. Be sure to wait until glassware is cool before transferring it
from place to place.
Lab16–Formula of a Hydrate
1
Procedure
1. Clean a porcelain crucible and dry it by gently heating it for about 1 minute. When it is cool, find
the mass of the crucible. Record it in Data Table 1.
2. Add 1.00 g of copper (II) sulfate · XH2O to the crucible. (You may be a little over or under, but
record the exact mass accurately.) Record the new mass of the crucible and its contents in Data Table
1.
3. Set up the crucible, clay triangle, and burner as shown.
4. Light the burner and adjust the flame so it just touches the crucible bottom. DO
NOT PUT THE CRUCIBLE TOO FAR INTO THE FLAME. YOU WANT TO
HEAT IT GENTLY.
5. Heat the crucible and its contents for 60 seconds. Swirl the flame underneath the
crucible to promote even heating. Shut off the flame and use crucible tongs to place
the crucible on the lab bench. Allow the crucible to cool completely. CAUTION:
Avoid touching the hot crucible.
6. Find the mass of the crucible and its contents. Record the mass of this first trial in Data Table 1.
7. Repeat steps 4 through 6 until at least two successive masses of the hydrate after heating are equal,
or your sample has completely changed color. You do not want to overheat your sample and
decompose the salt.
8. Clean up your work area and wash your hands.
DATA TABLE 1.
Mass of empty crucible
Mass of crucible + copper (II) sulfate · XH2O
Initial mass of copper (II) sulfate · XH2O
Trial # 1 Mass of crucible + hydrate after heating
Trial # 1 Mass of copper (II) sulfate · XH2O after
heating
Trial # 2 Mass of crucible + hydrate after second
round of heating
Trial # 2 Mass of copper (II) sulfate · XH2O after
heating
Trial # 3 Mass of crucible + hydrate after third
round of heating
Trial # 3 Mass of copper (II) sulfate · XH2O after
heating
Lab16–Formula of a Hydrate
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Analysis.
Use full sentences and show your work!
1. Discuss observations of the copper (II) sulfate salt before and after heating.
2. Calculate the mass of the water in the hydrated sample.
3. Determine the percent composition of the water in the hydrated salt.
4. Calculate the number of moles of anhydrous salt remaining.
5. Determine the number of moles of water evaporated from the hydrate.
6. Calculate the empirical formula for the hydrate.
7. Why did two successive mass reading have to be equal before finishing the experiment?
8. While you were waiting for your crucible to cool so you could weigh the anhydrous copper (II)
sulfate 2, the fire alarm went off for a fire drill. If the anhydrous copper (II) sulfate absorbed moisture
from the air, how would this affect your calculation for the number of moles of the anhydrous salt?
How would this affect the empirical formula calculation?
Lab16–Formula of a Hydrate
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