Community College of Rhode Island

400 East Ave.

Warwick, Rhode Island 02886

 

 

Name ________________________________

 

Lab Partner ____________________________

 

Chemistry Lab:  Analysis of a Hydrate—Part 1 

 

Objectives:     To find the formula of a hydrate.

                        To become more familiar with the properties of hydrates.

                        To review mass / mole relationships in chemical reactions.

                        To determine the percent water in an unknown hydrate.

 

Discussion:

            When an ionic solid is crystallized from water solution, the crystal which forms often contains chemically-bound water molecules.  The number of moles of water per mole of ionic substance is usually an integer.  Compounds of this sort are called hydrates.  Among the commonly encountered hydrates are the following:

                        CuSO4 . 5H2O             MgSO4 . 7H2O             BaCl2 . 2H2O

 

The water in a hydrate is bound loosely, and so is relatively easily removed by heating.  Most hydrates lose their water of hydration at temperatures slightly above 100oC.  Sometimes the water is liberated in stages, with one or more lower hydrates being observed during the heating process.  Thus, CuSO4 may also be prepared with 3 moles of H2O or 1 mole of H2O per mole of ionic solid.  If all the hydrated is removed, as it will be if the solid is heated sufficiently, the ionic solid is said to be anhydrous (without water).

            Given the mass of a sample of the hydrate (with water bound in the crystal) and the mass of anhydrous salt (without water bound in the crystal) of known formula obtained on heating, it is easy to find the formula of the hydrate.  One simply needs to determine the number of moles of water per mole of anhydrous compound in the hydrate:

 

Procedure:

 

1. Obtain a clean crucible.  Place the crucible on a clay triangle.  Heat with a bunsen burner for 2 minutes, gently at first and then strongly.  This will drive off any water that is adsorbed on the walls of the crucible.

 

2. Allow the crucible to cool on the triangle for 2 minutes.

 

3. When the crucible is cool, weigh it on a balance.  Make this and all successive weights to

+/- 0.01 g.  Record the weight in your data table.

 

4. Put about 3 g of the hydrate sample in the crucible (the amount doesn’t matter, what does matter is that you know how much you have).  Weigh the crucible with the hydrated magnesium sulfate.  Record the weight in your data table.

 

5. Place the crucible on the clay triangle on a ring stand.  Heat the crucible and its contents, gently at first and then strongly, for approximately 10-15 minutes.  The hydrate will probably have a different appearance when the water has been driven off.

 

6. Allow the crucible and contents (now an anhydrous solid (we hope)) to cool.  When they are at approximately room temperature, weigh the crucible and contents and record the mass in your data table.

 

 

Data Table:

Step 4                Mass of Crucible and Hydrated Magnesium Sulfate MgSO4-7H2O                    __________

                                                                       

Step 6                Mass of Crucible and Anhydrous Magnesium Sulfate MgSO4                __________

 

Step 3                Mass of Crucible                                                                                              __________

 --------------------------------------------------------------------------------------------------------------------------------

                        Mass of Water in Hydrated Magnesium Sulfate (H2O)                           __________

 

                        Mass of Anhydrous Magnesium Sulfate (MgSO4)                                              __________

_______________________________________________________________________________________

Results:

 

                        Number of Moles of Water Lost                                                                       __________

 

                        Number of Moles of Anhydrous Magnesium Sulfate                                          __________

 

                        Ratio:  Moles of Water / Moles of Anhy. Magnesium Sulfate                           :  1.00

                        (keep 3 sig figs in your ratio)

 

                        Relative Error (see next section for formula)                                                                      %

 

Calculations:  Neatly show and document, in your lab report,  the conversions and formulas you used to determine the answers you reported in the results section.  Messy calculations sections will not be read or given credit (this is where you document and prove your results)

 

                                                            | True Value - Lab Value  |

One time only:  Relative Error = ------------------------------------------  x 100

                                                                        True Value

 

(The true value for the ratio of moles water to moles magnesium sulfate is 7.00)

 

Conclusions:  Answer the following questions in complete sentences.

 

1. What would have happened to your Moles Water : Moles Compound ratio (larger, smaller, stay the same and explain why) if:

 

            a) you did not drive off all the water from the hydrate.

            b) you used a damp crucible and did not dry it before adding the hydrate.

            c) after the last heating you allowed the crucible and contents to cool overnight in the air

                before weighing.

 

ANALYSIS OF  PERCENT WATER IN AN  UNKNOWN HYDRATE---- PART 2

 

Using the assigned unknown, follow the above procedure and determine percent water in your unknown.

 

But instead of determining the empirical formula of your unknown you are determining the percent water attached to the compound.

The procedure is virtually the same except the percent water is determined by the mass lost (you can assume, unless you were sloppy, the mass lost=mass of water ) divided by the original mass times 100.

 

Your results and conclusion should state the percent water in the unknown compound that you had.