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Names: _______________________________________________________________
Lab Exercise: Percent Water in a Hydrate
Introduction: A hydrate is a crystalline solid that traps water as part of its crystal structure.
Each type of hydrate traps water in its own unique way, but heating a hydrate will release the
water and leave the dehydrated material behind. The solid remains unchanged except for
the loss of the water. In this exercise, you will weigh a sample of a hydrate, heat it to remove
the water, and weigh the dehydrated product that is left behind. Since you know the starting
amount, and the final amount, you can calculate how much water was driven off. Then you
will use your data to calculate the percent water in the hydrate.
Safety and waste disposal Information: Wear goggles when handling chemicals or
glassware. Be sure to let hot glassware cool before handling it. Dispose of your dehydrated
solid in the waste container provided. Wash your hands before leaving lab.
Procedure:
Wear goggles when handling glassware or chemicals!
Mass of the hydrate: Put a clean dry 30-mL beaker on the balance and re-zero by hitting
the O/T button so that it reads 0.0000. After all we don’t care about the mass of the beaker.
Remove the beaker from the balance, and sprinkle enough hydrate in the beaker to cover the
bottom. Put the beaker on the balance and record the mass of the hydrate by recording all
the numbers on the balance readout and the proper unit.
Heating the hydrate: Turn the “heat” dial on the hotplate to “9” and allow the hotplate to
warm up for a couple of minutes. Set the beaker containing the hydrate on the center of the
hotplate and observe carefully as the hydrate heats up. When all of the bright color of the
hydrate has disappeared, use crucible tongs to transfer the beaker to the desktop. Let the
beaker cool for at least 5 minutes.
Observations as the hydrate was heated (color, noise if any, and appearance)
Determining the mass of the dehydrated solid left behind. Put a second clean dry 30-mL
beaker on the balance and re-zero by hitting the O/T button so that it reads 0.0000. Remove
the second beaker from the balance, and transfer the dehydrated solid from the first beaker
into the second beaker. Put the second beaker on the balance and record the mass of the
dehydrated solid by recording all the numbers on the balance readout and the proper unit.
Calculating the percent water in the hydrate. Calculate the mass of the water driven off
as you heated the hydrate. Think about how to do this. What weights do you have? What
can you subtract to equal the weight of the water that was heated off? To calculate the
percent water in the original hydrate, divide the mass of the water driven off by the mass of
the hydrate and multiply that answer by 100 to convert it to a percent. Round off your answer
to the nearest tenth of a percent.
