Hello, Kailee,
The formula mass is the same as the molar mass. This is calculated by adding the AMU's for each element in one molecule of the compound, in the case CaCO3*6H2O. Make a table and count the number of times each element appears in the formula. Be careful here that you take into account the "*6H2O" in the formula. There are 6 water molecules tagging along on each calcium carbonate molecule. [That's just water's personality showing through.]
One might be tempted to ask why the formula isn't written as CaCO3(H2O)6 . The reason is that the water molecules aren't actually bonded to the CaCO3 molecule. They are simply attracted to it due to hydrogen bonding - not a bond in which electrons are taken (ionic) or shared (covalent), but a weaker bond of mutual attraction due to water's high polarity (it's two hydrogens stick mostly to one side of the molecule, making it very polar. Thus, the convention is "*6H2O." Since they are not chemically bonded, they can be convinced to leave the party when heat or even just dry air is present to dehydrate the hydrate, so to speak.
Getting back to the formula mass, make a table, count the elements, enter each element's atomic mass, and multiply each atomic mass by the number of each element found in the formula. Then sum those masses:
The formula mass for this hydrate is 208.1 AMU.
I hope this helps,
Bob
