Hey, this is Jessica, and TODAY we did something that will blow your mind. We were all assigned a hydrate of an unknown composition, and we used a bunsen burner to evaporate the water molecules. WOO it was so exciting.
Before I tell you about the lab, ther first thing you need to know is what a hydrate is. A hydrate is a compound that has a definite number of water molecules incorporated into its crystal structure. The crystals appear dry, but when these compouds are heated strongly, water is given off, leaving the anyhydrous form of the compound.
The first mass we found was the empty crucible. We weighed the crucible on a centigram scale. The mass my partner and I found was 25.23g. Then we found the mass of the crucible and the hydrate, which was 30.11g.
To find the mass of the hydrate alone, this is the formula:
Mass of crucible and hydrate - Mass of empty crucible = Mass of hydrate
30.11g - 25.23g = 4.88g
Next, we heated the crucible and anhydrous salt using a bunsen burner and weighed it again on the centigram scale. The mass we found was 27.79g. After that was found, we heated the same objects again for a second heating and came up with a mass of 27.78g
To find the mass of the anhydrous salt alone, this is the formula:
Mass of the first heating - Mass of the empty crucible = Mass of anhydrous salt
Lastly, to find the mass of the water given off, this is the formula:
Mass of the hydrate - Mass of the anhydrous salt = Mass of the water given off
ANALYSIS OF RESULTS (whopee)
1. Calculate the percentage of water in a hydrate
%H20 = 2.33g
---------- x 100% = 47.7%
2. Calculate the number of moles of the anhydrous salt left behind.
2.55g x 1mole
--------------- = 0.02 mol
3. Calculate the number moles of water removed by heat from your sample of hydrate.
2.33g x 1 mol
----------- = 0.13 mol
4. Calculate the moles of water per mole of the anhydrous salt.
------ -> 1
------ -> 7
5. What is the empirical dormula of the hydrate?
AB . 7H2O