ADVANCED PLACEMENT CHEMISTRY
NAME___________
DETERMINATION OF THE FORMULA OF A HYDRATE
Purpose: To determine the formula of a hydrate using two different methods.
Introduction
In the process of identifying an unknown compound or verifying the synthesis of a
compound, it is useful to determine the formula of the compound. There are many ways to do
this, and the following laboratory activity will illustrate one of these ways. The first step is will
involve the determination of the water of hydration.
Prelab
1. Determine the accepted value for the percent of water in any alum sample. The formula
for alum is KAl(SO4)2•12H2O. Show work.
2. A 1.000 g sample of alum is dessicated as described in part B of this lab. Once the
sample is completely dried, 0.593 g of anhydrous alum remains. Use this data to
determine the experimental formula for alum in the sample. Show work.
Procedure
Part A: Desiccation by a Bunsen Burner
1. Wash a crucible and lid. Dry them using a paper towel.
2. Set up a ring stand, iron ring, clay triangle, crucible and cover, and Bunsen burner as shown in
the figure at the bottom of page. Heat the crucible with its cover in the hottest part until it is
red hot, about 3 minutes. The inner blue cone should be approximately 3 cm below the
bottom of the crucible. Allow them to cool (about 3 minutes) and find the mass to the
nearest 0.01 g. [Once you have cleaned the crucible it is important to carry it using tongs.
This prevents burns and add will eliminate error caused by the weight of your fingerprints.
3. Add a few scoops of alum crystals to the crucible. Accurately determine the mass of the
crucible, cover, and crystals.
4. Return the crucible to the triangle and set the cover slightly ajar so the water vapor can escape.
For the first 3 minutes, use only VERY gently heat. Only upper the light blue portion of the
flame touches the crucible. Do this by holding the Bunsen burner.
5. Next, reposition the Bunsen burner so that the tip of the blue cone is approximately 3 cm
below the bottom of the crucible. Heat the crucible until it glows red for 10 minutes.
6. Remove the heat and completely cover the crucible. Allow it to cool to room temperature on
the triangle. Weigh the crucible, cover, and contents.
7. Once you have made you final weighing, invert the crucible to dump out the anhydrous alum.
You may dispose of it in the trash. Rinse and dry the crucible and cover.
Part B: Desiccation by a Drying Oven
8. Add a few more scoops of alum crystals to the crucible. Accurately determine the mass of the
crucible, cover, and crystals.
9. Place the crucible into the drying over on a paper towel labeled with your name. Leave this to
dry overnight.
10. Mass the contents of the crucible during the next class period.
Raw Data
Method
Bunsen Burner
Drying Oven
Mass of Crucible, Lid (g)
Mass of Crucible, Lid, Hydrated Alum (g)
Mass of Crucible, Lid, Anyhdrous Alum (g)
Calculated Data
Method
Mass of Hydrated Alum (g)
Bunsen Burner
Drying Oven
Mass of Anhydrous Alum (g)
Moles of Anyhydrous Alum (mol)
Mass of Water Lost (g)
Moles of Water Lost (mol)
Percent of Water Lost (%)
Accepted Percent Water Lost (%)
Percent Error (%)
Calculations
1.
2.
3.
4.
5.
6.
Calculate the mass of hydrated alum in your sample.
Calculate the mass of anhydrous alum in your sample.
Calculate the moles of anhydrous alum, KAl(SO4)2, in your sample using its molar mass.
Calculate the mass of water lost.
Calculate the moles of water lost.
Calculate the ratio of moles of H2O:moles KAl(SO4)2, and give the calculated formula of
alum, KAl(SO4)2•XH2O, where X = the ratio.
7. Calculate the percent water lost from hydrated alum sample.
8. Calculate the accepted percent water for alum using its formula for alum:
KAl(SO4)2•12H2O
9. Find the percent error for the percent water lost.