INTRODUCTION:

The strong dipole of water, which causes it to be an excelent solvent for ionic substances, also causes water molecules to attach themselves to ions in solution. Such ions are called Hydrated ions. When some solutions of hydrated ions are evaporated, the water molecules are so strongly attracted to the ions in solution that they remain attached as crystallization occurs. Water molecules are incorporated into the crystal structure. This water is called water of hydration.
Crystals that have formed in this way appear to be perfectly dry, yet when heated yield large quantities of water. The crystals change form, sometimes, even color, as the water is driven off. This indicates that the water was present as an integral part of the crystal structure. Such compounds are called hydrates. The number of moles of water present per mole of anhydrous salt is usually some simple number.
In this experiment you will be given an appropriate hydrate selected by your teacher. You will find the mass of water driven off by heating and the amount of anhydrous salt that remains. Your teacher will give you the formula of the anhydrous salt so you can find the empirical formula of the hydrate.

PREP NOTES FOR INSTRUCTOR:

This lab is set up over 2 days, with a pre-lab, to match a normal class time.

Suitable hydrates are: BaCl₂ * 2H₂O , MgSO₄ * 7H₂O , Na₂CO₃ * H₂O , CuSO₄ * 5H₂O , MnSO₄ * H₂O.

MATERIALS:

Safety goggles
Crucible and cover
Clay triangle
Pencil
Crucible tongs
Ring stand with ring
Burner
desiccator
Assigned hydrate

PROCEDURE:

PRE-LAB:

1. Record on your DATA TABLE, the formula of the anhydrous salt.

2. Record on your DATA TABLE, the formula weight of the anhydrous salt to four significant figures.

3. Record on your DATA TABLE, the formula weight of water to three significant figures.

DAY 1:

4. obtain a clean, dry crucible with cover , and using a pencil, mark the cover with an identifying mark. Place the crucible and cover in a triangle mounted on an iron ring and heat with a non-luminous flame for two or three minutes.

5. When the crucible and cover are cool enough to touch, transfer them to a balance. Find their mass to the nearest 0.01g. Record the mass on the DATA TABLE.

6. Obtain your assigned sample of the hydrate crystals and place them into the crucible. The crucible should be between one-fourth and one-third full. Replace the cover and find the mass. Record the mass on the DATA TABLE

7. Place the covered crucible on the triangle and heat gently, with a non-luminous flame, until most of the water has been driven off. Then increase the heat until the crucible bottom is at most a dull red. Maintain this temperature for five minutes. Using the crucible tongs, carefully transfer the crucible to the desiccator to cool.

DAY 2:

8. When the covered crucible is cool enough to touch, transfer it to the balance and find the mass. Record the mass on the DATA TABLE.

DATA TABLE:

Formula of the anhydrous salt. ____________

Formula weight of the anhydrous salt. ____________

Formula weight of water. ____________

Mass of the crucible and cover. ___________ g

Mass of crucible, cover, and sample before heating (hydrate). ___________ g

Mass of crucible, cover, and sample after heating (anhydrous salt). __________ g

CALCULATIONS:

9. Calculate the number of grams of the anhydrous salt you prepared. This is a simple subtraction from your data.

10. From the number of grams of the anhydrous salt in step 9, calculate the number of moles of the anhydrous salt you prepared.

11. How many grams of water were in your hydrate sample. This is the difference in mass, between the hydrate and the anhydrous salt.

12. How many moles of water were in your hydrate sample? Simply convert the grams of water in step 11 to moles.

13. How many moles of water were associated with 1 mole of the anhydrous salt? This is simply a ratio.

Number of moles of water divided by the Number of moles of anhydrous salt

REMEMBER, we can not use part of a water molecule, so round your answer to the nearest whole number.

14. From the answer to step 13, and the formula of the anhydrous salt, write the empirical formula for the hydrate.

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