After heating the crucible, cooling, and weighing the sample as 0.739 g, the students should have heated the crucible again and then reweigh the sample until a constant weight is obtained.
The sample being heated, CaCl2 2 H2O, contains water of hydration.
In order to arrive at an anhydrous sample of the salt, all the water molecules must be completely evaporated off by heating.
To do so, the sample should have been heated to constant weight by repeated heating and weighing until there is no longer a change in the weight of the sample.
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