EMPIRICAL FORMULA OF A HYDRATE

Many salts crystallized from water solutions appear to be perfectly dry, yet when heated, they liberate large quantities of water.  The crystals change form, even color, as the water is driven off.  Such compounds are called hydrates.  The number of moles of water present per mole of anhydrous salt (salt minus water of crystallization) is usually a whole number.  One example is the hydrate of copper (II) sulfate.  Its blue crystals look and feel dry.  Yet each mole of hydrate contains 5 moles of water.  Its formula is CuSO4 . 5 H2O.  The dot between the CuSO4 and the 5 H2O does NOT mean multiplication.  It indicates that 5 water molecules are bound to the other atoms.  The molar mass of CuSO4 . 5 H2O is 249.6 g/mole.

In this experiment, you will determine the empirical formula of a hydrate.  You will do so by heating the hydrate to drive off the water.  You will measure the mass of the water driven off the hydrate by heating it and you will measure the mass of the anhydrous salt that remains.  By calculating the number of moles of water driven off and the number of moles of anhydrous salt remaining, you will be able to find the empirical formula of the hydrate.

Materials
3 - 5.25" glass test tubes
ring stand, wire gauze, iron ring, & burner
hydrated salt
wood splint spatula for transferring solid to test tubes

Procedure
*PUT YOUR GOGGLES & APRON ON NOW!*
1.  Obtain and label three dry test tubes #1, #2, and #3.  Weigh the test tubes to the nearest 0.01 g.  Record these data on the data table below.  Remember to zero the balance each time you use it.  Also remember to use the same balance throughout the experiment.

2.  Add 0.5 - 0.8 g of the hydrate to each of the test tubes.  Do not pack the crystals.

3.  Reweigh the three test tubes and record the masses, again to the nearest 0.01 g.

4.  Lay the test tubes on the square wire gauze.  Turn the corners of the wire gauze up slightly so the test tubes do not roll off.  You must be able to heat the entire length of the test tube with the burner to drive off all the water of hydration.  Place the wire gauze and the three test tubes on the iron ring and heat the three samples gently and evenly along their entire lengths.

5.  Heat the test tubes by gently waving the burner flame back and forth underneath the wire gauze.  Record any observations you make during the heating process.

6.  Heat for approximately 5 minutes.  Allow the test tubes to cool before handling.  This may take several minutes.  (Remember that the glass is very hot even though it may appear to be cool.)

7.  Weigh the three samples and record the data.

8.  Reheat the three samples for an additional five minutes, cool, and reweigh until the mass of the anhydrous salt and test tube is constant.  What does “anhydrous” mean?

 

 

Trial 1

Trial 2

Trial 3

1

mass of empty test tube (g)

 

 

 

2

mass of hydrate & test tube (g)

 

 

 

3

mass after 1st heating (g)

 

 

 

4

mass after 2nd heating (g)

 

 

 

5

constant mass of anhydrous salt & test tube (final mass, g)

 

 

 

6

mass of water driven off (g)

 

 

 

7

# of moles of water

 

 

 

8

molar mass of anhydrous salt (______________)

 

 

 

9

grams of anhydrous salt (g)

 

 

 

10

moles of anhydrous salt

 

 

 

11

mole ratio of H2O to anhydrous salt

 

 

 

12

Empirical formula of hydrate -
experimental 

 

 

 

13

Empirical formula of hydrate -
theoretical

 

 

 

Calculations
Show all calculations necessary to calculate the empirical formula for the hydrated salt.  You have to show your work for only one trial.

Grading
To receive full credit, the data table must be filled in completely and correctly for all three trials.  You must include a sample calculation sheet along with your data table.

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