# The Percentage of Water in a Hydrate - WeissChem

The Percentage of Water in a Hydrate Mole Theory Unit Chemistry I Honors A Review of Percent Composition Remember that the percentage of any defined part of a system is defined as the total of that part divided by the total of all of the parts. The result of that division is then multiplied by 100 to give the final answer. The percentage of green will be calculated as: 3 green

X 16 total = 18.75 % 100 Now applying this to chemical compounds Example: What is the percentage of oxygen in the compound sodium sulfate? Sodium sulfate has

the formula Na2SO4 4 ( 16 ) grams O [ 2(23) + 1(32.1) + 4(16) ] g Na2SO4 = The percentage of oxygen will be calculated as: 64 X

100 142.1 = 45.04 % oxygen Now for: What is a Hydrate ??? These are compounds that consist of a regular ionic compound with some number of water molecules attached to

the perimeter of that ionic molecule. H2O H2O H2 O Ionic molecule H2O H2O We can describe the compound as having a generic formula with the format:

AX n H2O Represents the ionic molecule a coefficient The dot means attached to The H2O is called the water of hydration

Naming these hydrates: There will be two components to the name and the name will end up having three words: Name the ionic molecule following the rules for any other ionic compound. Write a covalent prefix for the number in the coefficient spot and attach the word hydrate. Example: The compound has the formula MgCl2 6 H2O Its name is magnesium chloride hexahydrate Formula Weight of Hydrates: You will still determine the formula weight using the formula

f.w. = (inventory x atomic weight) The only difference is that you will have to include the mass of the water molecules in the total. It will help to remember that water has a formula weight = 18 grams/mole Example: Determine the formula weight of the compound: Na3PO4 4 H2O Solving: fw = 3(23) + 1(31) + 4(16) + 4(18) = 236 grams/mole Note that in this case the dot means attached to so we will ADD the mass of the water molecules. (we will not multiply like in math class)

Now for Percentage of Water You can see that the water molecules make up an identifiable part of the complete compound. Therefore, you can determine the percentage of water in a given hydrate. It will be calculated as: % water = Total mass of water X 100 Total mass of hydrate An example Calculate the percentage of water

in iron (III) chloride trihydrate Step #1 write the formula FeCl3 3 H2O Step #2 set up the calculation % H2O = = 3(18) 1(55.8) + 3(35.5) + 3(18) 54 216.3

X 100 X 100 = 24.97 %

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