Formula of a Hydrate

Formula of a Hydrate

Description

A sample of hydrated compound is weighed. It is then heated to drive off moisture and reweighed to constant weight. All of the mass loss is assumed to be due to water of hydration. From the initial and final masses, the formula for the hydrate is determined.

Precautions

Use care handling hot glass and porcelain objects. Place fingers near the object to sense heat.
Eye protection is particularly important; there is a possibility of spattering if the water leaves the solid too quickly.

Procedure

Heat a crucible (or test tube) briefly to drive off any volatile materials and then cool.
Weigh the crucible (including a cover), or the test tube (with a stopper).
Add approximately 2 grams of hydrate, and reweigh. Record.
Heat the hydrate gently over a Bunsen burner for 3-5 minutes. Keep the crucible lid ajar while heating. Remove the stopper when heating a tube. A definite color change will be noted if the hydrate is copper sulfate. If a test tube is used, heat along a slanted tube so the maximum surface of the hydrate is exposed to the heat.
After heating, turn off the gas. Using tongs, remove the lid and then the crucible. Place on a wire gauze or other pad. Allow to cool. Cover the crucible with the lid while it cools. (Stopper very loosely when using a test tube.)
Cool the crucible and lid until it can be held comfortably.
Weigh it.
Use a stirring rod to crush the crust on the heated material. Reheat and cool. Reweigh. If a mass change has occurred, heat again.
Repeat the heating, cooling, checking, and weighing procedures until two successive weighings agree with one another.
This test for reaction completion is called heating to constant weight.
If the crucible must be stored between periods, place it in a container with CaCl₂ to maintain the water loss or place it in a drying oven. A standard desiccator or a coffee can with a screen over the CaCl₂ will be fine for storage.
After the dehydrated salt has been weighed a final time, place a thermometer into the dehydrated salt and record the temperature.
Add three or four drops of water at room temperature to the salt and record the final temperature of the system.

Handout

Name ___________________________ Class ________

Teacher__________________________

DoChem 018 Formula of a Hydrate

Mass of empty crucible and cover (test tube and stopper) =
Mass of apparatus and hydrate =
Mass of apparatus and salt after 1st heating =
Mass of apparatus and salt after 2nd heating =
Mass of apparatus and salt after 3rd heating (if necessary) =
Mass of hydrate =
Mass of water =
Mass of dehydrated salt =
Temperature of anhydrous salt after cooling =
Temperature of system after water has been added =

Calculate the number of moles of hydrate used; anhydrous salt produced; and water lost by the reaction.
Set up an empirical formula for the hydrate by putting the whole number mole ratio of water to anhydrous salt in place of the x in the formula CuSO₄•xH₂O
List a few possible sources of error in this experiment, and state whether each would have caused a high or low value for x in the reported formula.
Is the process of removing water from a hydrate endothermic or exothermic? What about the reverse reaction?

Handout Makeup

Name ___________________________ Class ________

Teacher__________________________

DoChem 018 Formula of a Hydrate

Watch the movies, and answer both sets of questions.

What is meant by a hydrate.
Why is the formula of the water in a hydrated compound separated by a dot (e.g. BaCl₂•2H₂O) instead of being included in one formula (BaCl₂H₄O₂)?
Why is the hydrate heated at least twice?

Use this sample data to answer the questions below.

Mass of empty crucible and cover = 24.32 g
(test tube and stopper)
Mass of apparatus and hydrate = 26.75 g
Mass of apparatus and salt after 1st heating = 25.93 g
Mass of apparatus and salt after 2nd heating = 25.88 g
Mass of apparatus and salt after 3rd heating (if necessary) = 25.88 g
Mass of hydrate = 2.43 g
Mass of water = 0.87 g
Mass of dehydrated salt = 1.56 g
Temperature of anhydrous salt after cooling = 27 °C
Temperature after water added = 89 °C

Calculate the number of moles of hydrate used; anhydrous salt produced; and water lost by the reaction.
Set up an empirical formula for the hydrate by putting the whole number mole ratio of water to anhydrous salt in place of the x in the formula CuSO₄•xH₂O
List a few possible sources of error in this experiment, and state whether each would have caused a high or low value for x in the reported formula.
Is the process of removing water from a hydrate endothermic or exothermic? What about the reverse reaction?

Teachers Guide

Purpose

To determine the amount of water in a hydrate.
To determine the formula of the hydrate.

Materials

(Per 10 students working in pairs)

5 crucible and cover or test tube (with stopper and suitable tube clamp)
5 support stand and ring
5 pairs crucible tongs
5 clay triangle
5 centigram balance
10 grams of hydrate (CuSO₄•5H₂O)
5 Bunsen burner, ignition source
5 thermometer
5 wire gauze or other pad for hot objects
5 wash bottle with distilled water
5 scoopula
5 20-cm long x 5-mm diameter fire-polished glass stirring rod

Lab Hints

Crucibles are very fragile.
Nickel crucibles are more expensive initially but avoid the problems of breakage and damage to the ceramic glaze by high temperature reactions.
Students should be encouraged to heat the hydrate at least twice, weighing each time, to be sure the water has been removed completely. If the weight change is large after two heatings, a third heating may be required.
Stress that the lab must be done in one period or the anhydrous salt will rehydrate overnight.
Care must be taken to avoid decomposition of the anhydrous salt indicated by a yellowing in the CuSO₄ or production of an odor.

Time

Teacher preparation: 15 minutes

Class Time: 45-50 minutes (Some students may only do one drying and weighing step in this time. IF you wish students to dry to a constant weight, the experiment will require two periods. You will need some sort of desiccator or drying oven to store solids between periods.)

Hazards

The crucibles may cause burns to students or damage to balances if not cooled properly before weighing. The heated materials may spatter.

Precautions

Caution students about handling hot glass and porcelain objects. Place fingers near the object to sense heat.
Eye protection is particularly important; there is a possibility of spattering if the water leaves the solid too quickly.

Disposal

In most communities the copper compounds may be dissolved and disposed of by flushing down the sink.

Set?

Set Questions:

What is meant by a hydrate.
Why is the formula of the water in a hydrated compound separated by a dot (e.g. BaCl₂•2H₂O) instead of being included in one formula (BaCl₂H₄O₂)?
Why is the hydrate heated at least twice?

Answers to Set Questions:

A hydrate is a substance with "trapped" water weakly bonded to it.
The water remains as discrete water molecules within the solid substance. The water is a part of every sample, but it is an easily removed part due to weak bonding forces.
If the second heating produces the same mass as the first, the water is gone and there is no need to heat further.

Sample Data

Mass of empty crucible and cover = 24.32 g
(test tube and stopper)
Mass of apparatus and hydrate = 26.75 g
Mass of apparatus and salt after 1st heating = 25.93 g
Mass of apparatus and salt after 2nd heating = 25.88 g
Mass of apparatus and salt after 3rd heating (if necessary) = 25.88 g
Mass of hydrate = 2.43 g
Mass of water = 0.87 g
Mass of dehydrated salt = 1.56 g
Temperature of anhydrous salt after cooling = 27 °C
Temperature after water added = 89 °C

Closure?

Closure Questions:

Calculate the number of moles of hydrate used; anhydrous salt produced; and water lost by the reaction.
Set up an empirical formula for the hydrate by putting the whole number mole ratio of water to anhydrous salt in place of the x in the formula CuSO₄•xH₂O
List a few possible sources of error in this experiment, and state whether each would have caused a high or low value for x in the reported formula.
Is the process of removing water from a hydrate endothermic or exothermic? What about the reverse reaction?

Answers to Closure Questions (based on sample data):

26.75 g - 24.32 g = 2.43 g CuSO₄•5H₂O;

2.43g CuSO₄•5H₂O x (1 mol CuSO₄•5H₂O/249.5g CuSO₄•5H₂O) = 0.00974 mol CuSO₄•5H₂O;
25.88 g - 24.32 g = 1.56 g CuSO₄;
1.56 g CuSO₄/159.5 g/mol = 0.00978 mol anhydrous salt;
26.75 g -25.88 g = 0.87 g water;
0.87 g/18.0 g/mol = 0.048 mol water.
(0.048 mol H₂O)/(0.00978 mol CuSO₄) = 4.9≅5
Not removing all of the water from the hydrate would produce a low value for x; overheating and decomposing the salt would produce a high value for x; loss of anhydrous salt during handling of test tubes would produce values of x larger than the predicted value.
Adding water to the anhydrous salt released heat and therefore was an exothermic reaction. The reverse process is endothermic.

Applications

Students have often seen little packets of silica gel packed with vitamins, cameras, stereo equipment, etc., to avoid damage from moisture. Anhydrous salts are often used for purposes such as these. They are reusable in that heating will reactivate one that is hydrated fully.
Many purified chemicals (e.g., NaOH and H₂SO₄) must be carefully sealed from the atmosphere because they so easily pick up water of hydration.

Computer Use

Students may enter the class' data directly into a worksheet of a spreadsheet computer program.

Makeup Ans.

Questions are Set questions followed by the Closure questions. Answers are the same.

Key Words

hydrate
water of hydration
formula
mole
energy of hydration
heating to constant weight