The Magic of Hydrates!


 

In lab 4C we were taught how to find the percentage of water in an unknown hydrate! Cool, eh? And it was simple too. First, you may be wondering what a hydrate is... 

-A hydrate is a compound/substance containing water molecules.



The hydrate

CuSO₄ • 5H₂O

When the hydrate is heated and the water is evaporated, it becomes an anhydrous salt.

-An anhydrous salt is a salt containing no water.

 

The anhydrous salt

CuSO₄ • 5H₂O

Alas, let's start the lab!!

Materials:

-lab burner                                                

-crucible

-crucible tongs

-pipestem triangle

-ring stand & ring

-centigram

-safety goggles

Chemical Reagents:

-approx. 5 g of a hydrate

Process:

1.  Put on safety goggles and tie any loose hair back. Remember; safety first!
2. Set up your ring stand (screw on the iron ring to the stand, then put the pipestem triangle on top of the ring.) Also, plug in your Bunsen burner into the gas valve. The iron ring should be about 5 cm above the Bunsen burner.
3.  Once you have your Bunsen burner and ring stand set up, place the crucible on the pipestem triangle. Heat the crucible for 3 minutes to ensure it is dry. *your fire should be blue, NOT orange! If its orange, it'll release carbon bits on to your crucible!*
4. Now, remove the crucible off the burner with your crucible tongs and let it cool. To check if it's cool, bring the back of your hand close to the crucible. If you can still feel heat, let it cool more!
5. Measure the mass of the crucible and record it
6. Put some of the hydrate into the crucible until it's about 1/4 full. Then, weigh it again. Record the mass of the hydrate.
7. Place the crucible back on the pipestem triangle and heat it for about 5 minutes.
8. Turn off the burner off and allow the crucible to cool. (Remember to use the backk-of-the-hand trick to determine whether it's cool or not). Once its cooled, record the mass of the crucible and the hydrate.
9. Do another heating. This is to ensure ALL the water is evaporated. Heat the crucible again for 5 minutes, let it cool, then record your masses. (The two masses should be 0.03 g apart at maximum.)
10. After having recorded all your masses, cool the crucible and add a drop of water back into the crucible. Record any changes the anhydrous salt undergoes.

Record Table:

Mass of empty crucible
Mass of crucible and hydrate
Mass of hydrate
Mass of crucible and anhydrous salt (first heating)
Mass of crucible and anhydrous salt (second heating)
Mass of anhydrous salt
Mass of water evaporated
Mass of one mole of anhydrous salt
Describe any changes after adding a drop of water

How do we find the % of water given off?
Well, that's simple! Subtract the mass of the anhydrous salt from the mass of the hydrate. Then divide the mass of water given off by the mass of the hydrate. Multiply by 100 to get the percentage of water burned off. To get the mass of the salt, divide the mass of the salt by the mass of the hydrate. *A good way to see if your calculation is right is to add the percentages together-they should add up tp approximately 100.*

Example:

If you have a hydrate that weighs 3.20 g and an anhydrous salt that weighs 1.95 g, what is the percentage of water given off?

3.20 g   <--mass of hydrate
1.95 g   <--mass of anhydrous salt
1.25 g   <--mass of water

1.25 g  
3.20 g  = 0.399625

0.399625 x 100 = 40.0% - Don't forget sig figs!

The percentage of water given off is 40.0%

1.95 g   <--anhydrous salt
3.20 g = 0.609375

0.609375 x 100 = 60.9 %

The percentage of the anhydrous salt is 60.9%

40.0 + 60.9 = 100.9  <-- the mass of the anhydrous salt and water is ~100, therefore the answer is correct!