What the hell is percent composition of hydrates?!?!?

Language, young man/woman!

Okay, here's what you need to know about percent composition of hydrates: A hydrate is a compound that contains water molecules locked up into its crystal structure. A common hydrate is copper(II) sulfate pentahydrate, or CuSO4(H2O)5. Each mole of copper(II) sulfate has 5 moles of water bound to it.

Let's say you wanted to find the percentage of CuSO4(H2O)5 that is made of water, by mass. First you have to find the molar mass of CuSO4(H2O)5, then you have to determine what percentage of that mass is made of water:

Molar mass of CuSO4(H2O)5:

1 x Cu = 63.55

1 x S = 32.07

4 x O = 4 x 16.00 = 64.00

5 x H2O = 5 x 18.02 = 90.10

Total = 249.72 g/mol

Now we know that water comprises 90.10 grams out of every 249.72 grams of copper(II) sulfate pentahydrate, so...

%H2O = 90.10 g / 249.72 g x 100 = 36.08%

So for any given mass of CuSO4(H2O)5, 36.08% of that mass is made of water. The remaining 63.92% is made of CuSO4.

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Here's another example: Sodium carbonate heptahydrate, or Na2CO3(H2O)7. What percentage of this compound is made of water?

Step 1: Calculate the molar mass of Na2CO3(H2O)7

2 x Na = 2 x 22.99 = 45.98

1 x C = 12.01

3 x O = 3 x 16.00 = 48.00

7 x H2O = 7 x 18.02 = 126.14

Total = 232.13 g/mol

Step 2: Calculate the percentage of water.

%H2O = 126.14 g / 232.13 g x 100 = 54.340%

So for any given mass of sodium carbonate heptahydrate, 54.340% of that mass is made of water molecules. The remaining 45.660% is made of Na2CO3.

I hope that helps. Good luck!