chemistry 2

Percentage of water in a hydrate

Background:

The accurate determination of the composition of substances is essential in modern

technology and important in our everyday lives. Analytical chemistry plays a role in

determining the compositions of potentially valuable ores, monitoring the composition of acid

rain and detecting contamination in food stuffs.

Analytical measurements may be either qualitative or quantitative. Qualitative

measurements involve finding out the identity of an unknown substance. Quantitative

measurements involve determining the amounts of known substances in various samples.

In this experiment you will work on finding the amount of water in a hydrate. A hydrate

is a solid material that incorporates water molecules in the crystal lattice. The presence of the

water molecules makes the molar mass of a hydrate higher than that of the anhydrous (meaning

without water) version of the substance. For example aluminum nitrate nonahydrate has the

following formula: Al(NO3)3•9H2O. Anhydrous aluminum nitrate has the following formula

Al(NO3)3.

Experimental Procedure:

SAFETY PRECAUTIONS

WEAR SAFETY GOGGLES DURING ALL HEATING OPERATIONS.

Obtain a small beaker (~30mL). Clean it with de-ionized water and dry with a paper

towel. If the beaker is still not clean, add 5-10mL of 6M HCl and allow it to stand for about 5

minutes. Rinse with de-ionized water and re-dry. After the beaker has been cleaned, use tongs to

handle it.

From this point on do not touch the beaker with your bare fingers.

Place the beaker on a wire gauze with a ceramic center. Heat gently with your Bunsen

burner for a minute or two and then at full flame intensity for 5 minutes.

Remove the flame and allow the beaker to cool to room temperature.

When the beaker is completely cool, use tongs to move it to an asbestos square or flat

glass plate. Do not place it directly on the lab bench. Weigh the beaker to the nearest tenth of a

milligram (0.0001 g) and record this weight.

Return the beaker to the wire gauze. Reheat in the full heat of the burner flame for 5

minutes. Cool and re-weigh. If the two weights do not agree within 5 mg (0.0050 g), continue

heating/weighing until they do. Record the final dry weight of the beaker.

Weigh out between 2.2500 and 2.5000g of the copper (II) sulfate pentahydrate,

CuSO4•5H2O, on weighing paper. Carefully transfer the hydrate to the beaker and re-weigh.

Record this weight.

Set the beaker on the wire gauze. Heat the beaker gently for a few minutes and then

adjust the Bunsen burner to heat the beaker with the full heat of the burner flame for 15 minutes.

If the contents of the beaker begin turning yellow, reduce the size of the flame.

Allow the beaker and contents to cool completely to room temperature. When completely

cool, weigh the beaker and contents to the nearest tenth of a milligram (0.0001 g). Record the

weight.

Return the beaker to the wire gauze and heat for another 5 minutes in the full heat of the

burner flame. Cool and re-weigh. The two weights should agree to within 5 mg (0.0050 g). If

they do not, continue heating and weighing until they do.

Once your beaker and residue have reached constant weight, record the weight and add a

few drops of water to the residue. Record your observations.

The residue may be disposed of in the trash can.

Calculations:

Using your data calculate the following:

1. mass of the hydrate before heating.

2. mass of the residue in the beaker.

3. mass of the water removed from your hydrate sample.

4. molar ratio of water to copper(II) sulfate

5. percentage by mass of water in your sample

6. theoretical percentage by mass of water in copper(II) sulfate pentahydrate

7. percent error of your percentage by mass of water

%100 

 valueaccepted

valueacceptedvalueyour errorPercent

Date _____________ Section ________ Name _________________________

Data and Calculations Sheet: Percentage of water in a hydrate

Data:

Mass of empty beaker (after first heating) __________ __________

Mass of empty beaker (after final heating) __________ __________

Mass of beaker with hydrate __________ __________

Mass of beaker & residue (after first heating) __________ __________

Mass of beaker & residue (after final heating) __________ __________

Calculations: You MUST show your work on another sheet. If you are submitting a hard

copy lab report include the sheet with your work. If you are entering your data into

blackboard you should include the work for each result that you enter.

Results of calculations:

1. Mass of hydrate __________ __________

2. Mass of residue __________ __________

3. Mass of water lost __________ __________

4. Molar Ratio of water to hydrate (use your data) __________ ___________

5. % water in the CuSO4•5H2O (use your data) __________ __________

6. Theoretical % water in the CuSO4•5H2O __________

7. % Error __________ __________

Post Lab Questions:

1. While carrying the beaker and dried sample to the balance for the final weighing a student

unknowingly spilled some of the residue out of the beaker. Would the % water be too high or too

low? Explain your reasoning.

2. In the experiment you were instructed to allow your sample and beaker to cool to room

temperature before you completed your final weighing. A conscientious student allowed the

beaker to “cool” for a full week. Can you see any problems which might be caused by this

change in procedure? Explain.

3. What did you observe when you added water to the residue in the beaker? Can you explain

why it happened?

Date _____________ Section ________ Name _________________________

ADVANCED STUDY ASSIGNMENT: Determination of the percentage of water in a

hydrate

1. What does a chemist mean by the term hydrate as used in the context of ammonium oxalate

monohydrate?

2. Calculate the percentage by mass of water in magnesium sulfate heptahydrate, MgSO4•7H2O.

3. In an experiment 34.8243 grams of copper(II) nitrate hydrate, Cu(NO3)2•zH2O was heated to

a constant mass of 27.0351g.

a. What is the mass of water lost?

b. Calculate the percent water in this hydrate.

c. What is the value of z?