Chemistry I—Lab: Determining the Chemical Formula of a Hydrate

Purpose: The purpose of this lab is to ensure that the student understands the procedure of heating to constant mass and to determine from experimental data the number of moles of water released by a hydrate and infer the chemical formula of the hydrate.

Introduction: Many ionic compounds, when crystallized from aqueous solution will integrate definite amounts of water as a part of their crystalline structures. This water may be driven off by heating the compound to constant mass and it’s anhydrous form. Because the law of definite composition applies, the number of moles of water driven off per mole of anhydrous compound is a simple whole number. If the formula of the anhydrous compound is known, you can determine the formula of the hydrate.

Materials:

centigram balance Bunsen burner and tubing

crucible and cover desiccator

iron ring and ring stand Epsom salt

pipe stem triangle sparker

spatula crucible tongs

Procedure:

1. Always handle the crucible and cover with CLEAN crucible tongs!!!!

Place the crucible and cover on the triangle as shown in the figure below.

Position the cover slightly tipped, leaving only a small opening for gases

to escape. Preheat the crucible and cover to redness. The crucible and

cover are very hot after each heating. Always handle ONLY with


tongs!!!

2. Using tongs, transfer the crucible and cover to a desiccator. Allow to

cool for 5 min in the desiccator. Never place a hot crucible on a balance.

When cool, determine the mass of the crucible and cover to the nearest

0.01 gram. Record this mass in your data table.

3. Using a clean spatula, add approximately 5 grams of Epsom salt crystals

(magnesium sulfate hydrate) to the crucible. Record the mass of the

crucible, cover and Epsom salt. Determine the mass of only the Epsom

salt.

4. Place the crucible with the Epsom salt on the triangle and position the

cover so there is a small opening. Too large an opening will allow the

hydrate to spatter out of the crucible. Heat the crucible VERY GENTLY

with a low flame to avoid spattering! Increase the temperature gradually

for 2 – 3 minutes. Then heat strongly, but NOT RED HOT, for at least 5

min.

5. Using tongs, transfer the crucible, cover and contents to the desiccator

and allow to cool for 5 min. Be sure the crucible is sufficiently cool

before weighing. Record the mass in your data table.

6. Again heat the crucible (repeating step 4) for 5 minutes. Allow to cool in

the desiccator for at least 5 minutes and record the mass. Continue to

repeat this step until the masses differ by no more than 0.01 gram.

Record this constant mass in your data table.

7. Clean up all materials and dispose of the Epsom salt in the trash

container.

Calculations:

1. Determine the mass of anhydrous magnesium sulfate:

Answer = constant mass – mass of empty crucible and cover

2. Determine the number of moles of anhydrous magnesium sulfate.

3. Determine the mass of water driven off from the hydrate.

Answer = Mass of crucible and cover and UNHEATED salt hydrate –

constant mass

4. Determine the number of moles of water driven off from the hydrate.

5. Write the chemical equation to the loss of water from the hydrated salt.

Use the letter n to represent the number of moles of water driven off per

mole of anhydrous magnesium sulfate.

6. Use your answers to 2, 4 and 5 to determine the mole ratio of magnesium

sulfate to water to the nearest WHOLE number ratio.

7. Write the formula for the magnesium sulfate hydrate (Epsom salt).

Data Table:

Mass of empty crucible and cover
Mass of crucible, cover and magnesium sulfate hydrate (before heating)
Mass of crucible, cover and anhydrous magnesium sulfate after 1st heating
Mass of crucible, cover and anhydrous magnesium sulfate after 2nd heating
Mass of crucible, cover and anhydrous magnesium sulfate after 3rd heating
Constant mass of crucible, cover and anhydrous magnesium sulfate
Mass of anhydrous magnesium sulfate
Moles of anhydrous magnesium sulfate
Mass of water driven off from hydrate
Moles of water driven off from hydrate
Mole ratio of water to anhydrous magnesium sulfate
Empirical (chemical) formula of the hydrate

POST-LAB QUESTIONS AND DATA ANALYSIS

1.  Why must objects be cooled before their mass is determined on a sensitive balance?

2.  How many moles of water were in your sample of magnesium sulfate hydrate?

3.  How many moles of magnesium were in your sample of magnesium sulfate?

4.  How many moles of sulfate were in your sample of magnesium sulfate?

5.  Write the proper chemical formula and name for the compound that you tested, included the quantity of hydrate.

6.  A student fails to place the lid on the crucible during the initial heating of the hydrated sample and some of the solid spatters out. What effect does this error have on the calculated mass of the water lost by the hydrate? Justify your answer.

7.  Considering the following reaction: magnesium sulfate reacts with copper to yield magnesium and copper(II) sulfate

If 11.5 grams of magnesium sulfate react with 7.25 grams of copper, what will be the theoretical mass of copper(II) sulfate that will be produced?

If 13.4 grams of CuSO4 is actually recovered in lab, what would the percent yield for this quantity be?