Determine the Formula of a Hydrate

Procedure Summary & Vocabulary

It is important to know the rough sequence of steps for this lab for the AP curriculum. Below is the outline of steps involved in this lab:

• Take mass of empty, clean crucible

• Take mass of crucible with hydrated sample

• Heat sample in crucible

• Cool and mass the sample after heating

• Heat again and then cool and mass sample

• Repeat heating and cooling until the mass of sample remains constant

Below are important terms use within this lab:

• Hydrate = compound with water

• Anhydrous = compound without the water, sometimes known as “the salt”

• Efflorescence = the property of some substances to lose wholly, or partly their water of crystallization when their crystals are exposed to dry air even for a short time

• Desiccants =  a hygroscopic substance used as a drying agent.

• Hygroscopic = readily taking up and retaining moisture.

• Delinquency = the process by which a substance absorbs moisture from the atmosphere until it dissolves in the absorbed water and forms a solution

Chemical/ Material Preparation Notes

Optional Procedure Modifcations

The following are possible other hydrates that can be used in this experiment:

• BaCl22H2O

• MgSO4 7H2O 

Some procedures of similar investigations may call for the crucible to be heated before the beginning of the lab and/or use of a desiccator. This is not needed and is usually used when you are looking to be as precise as possible. You can still get good results by skipping the aforementioned steps. 

Some procedures may not call for a lid. The purpose of the lid is to avoid splattering of the solid. Regardless, when a lid is used, it is important that it is NOT completely covering the top. There needs to be an opening to allow the water to escape. (That is the purpose of heating the crucible!)  

Below are some standard questions that students can complete to prepare for the lab work:

Question(s) related to: Related Chemical Concepts

1. Complete the following table. For each of the hydrates: (a) write the chemical formula; (b) calculate the formula mass; and (c) calculate the mass percentage of water. 

Question(s) related to: Sample Data 

1. A student is given a cobalt (II) chloride hydrate. He weighs a clean and dry crucible with its cover and records a mass of 18.456 g. He then weighs the sample in the crucible and cover and obtains a mass of 19.566 g. He heats the sample, allows it to cool to room temperature and reweighs it to obtain a mass of 19.062 g. In the process, the sample’s color changed from red- burgundy to blue.

1. Mass of hydrate

2. Mass of anhydrous CoCl2

3. Mass of water driven off

4. Moles of water

5. Moles of anhydrous CoCl2

6. Moles of water per mole of CoCl2

7. Formula of hydrate

Solution to Questions Above:

1. Calculate the mass of the hydrate.

Mass of hydrate = Mass of crucible + cover + hydrate - Mass of crucible + cover

Mass of hydrate  =  19.566 g - 18.456 g  = 1.110 g

2. Calculate the mass of anhydrous cobalt(II) chloride.

Mass of anhydrous CoCl2 = Mass of crucible + cover + anhydrous CoCl2 - Mass of crucible + cover

Mass of anhydrous CoCl2 = 19.062 g - 18.456 g  = 0.606 g

1. Calculate the mass of water driven off.

Mass of water driven off = Mass of hydrate - Mass of anhydrous CoCl2

Mass of water driven off = 1.110 g - 0.606 g  = 0.504 g

1. Calculate the moles of water.

Molar mass of water (H2O) = 18.015 g/mol

Moles of water = Mass of water driven off / Molar mass of water

Moles of water = 0.504 g / 18.015 g/mol  = 0.028 moles

1. Calculate the moles of anhydrous cobalt(II) chloride.

Molar mass of CoCl2 = 129.84 g/mol

Moles of anhydrous CoCl2 = Mass of anhydrous CoCl2 / Molar mass of CoCl2

Moles of anhydrous CoCl2 = 0.606 g / 129.84 g/mol  =  0.00467 moles

1. Calculate the moles of water per mole of CoCl2.

Moles of water per mole of CoCl2 = Moles of water / Moles of anhydrous CoCl2

Moles of water per mole of CoCl2 = 0.028 moles / 0.00467 moles  ≈   6 moles

1. Determine the formula of the hydrate.

The formula of the hydrate indicates the number of water molecules per mole of the anhydrous compound. Since there are approximately 6 moles of water per mole of anhydrous CoCl2, the formula of the hydrate is CoCl2·6H2O.

Question(s) related to: Procedure/ Safety

1. Define a hydrate. 

2. Describe how to handle the crucible during the lab investigation and explain why.

3. In order for your mass measurements to be accurate, the crucible must be completely cooled. Why? What effect would a hot crucible have on your measurements?

4. How can you tell visually that the water escaped from the hydrate?

5. How many times does the hydrate need to be heated?

Solutions to Questions Above

1. Hydrates are compounds that have one or more water molecules integrated into their structure. 

2. The crucible should always be handled with crucible tongs throughout the lab. The tongs are used because the crucible will be strongly heated BUT ALSO to avoid fingerprints or moisture from touching the container itself. The potential moisture and/or transfer with fingers can impact the results. 

3. It is important that a crucible is cooled before taking its mass because the mass of the crucible can be affected by residual heat. When a crucible is heated, it expands, and this expansion can lead to an increase in its apparent mass. If the crucible is not allowed to cool completely before weighing, the residual heat can cause air currents around the balance, which can result in fluctuations in the mass reading.

4. The appearance of the solid changes color from blue to white. 

5. The hydrate must be heated at least 2 times, but the actual number can be several more. The number of heatings depends on two things: (1) the color change (if anhydrous form is a different color; that should be observed) and (2) after repeated heatings the mass remains consistent/ less than a few hundredths of a gram difference.

Sample Data

Sample Analysis/ Calculations

There are several hydrates that can be used to conduct this lab. For this version, the hydrate in question is:

CuSO4xH2O → CuSO4 + x H2O

1. Mass of hydrated sample = (Mass of crucible + sample) - (Mass of empty crucible) 

= 25.25 - 22.68 

= 2.57 grams

1. Mass of water in hydrate = (mass of crucible + hydrate) - (mass of sample after heating)

= 25.25 - 24.23

= 1.02 grams

There is 1.02 g of H2O in the sample of CuSO4xH2O used in this lab. 

Mass of salt/ anhydrous (hydrate without water) = (mass crucible + sample after heating) - (mass of crucible)

= 24.23 - 22.68

= 1.55 g

There is 1.55 g of CuSO4 in the sample of CuSO4xH2O used in this lab. 

1. Convert water to moles:

1.02 H2O  x  1 mol H2O18.02 g H2O   = 0.05660 g H2O

Convert anhydrous salt to moles:

1.55 g  CuSO4  x  1 mol CuSO4159.60 g CuSO4 =  0.009712g g  CuSO4 

1. Calculate ratio/ Formula of the hydrate:

mol H2Omol CuSO4  =  0.05660 mol 0.009712 mol =  5.78  → round to the nearest whole number = 6

Therefore, the hydrate’s formula was experimentally determined to be: CuSO46H2O (copper(II) sulfate hexahydrate)

Discussion Questions

What effect on the mass of the residue would occur if overheated and the compound started to decompose? Will this lead to a higher or lower value of the calculated amount of water in the sample?

If the solid began to decompose, assuming that meant the mass became less in the crucible than it should be, would mean that extra loss of mass would be calculated as additional water leaving the crucible, which actually did not. Therefore the amount of water in the sample would be calculated as higher than it actually should be. 

1. What is the effect of not heating the hydrate enough to drive off all the water in the hydrate? Will this lead to a higher or lower value of the calculated amount of water in the sample?

If the hydrate was not heat enough to drive off all of the water, then the calculated amount of water would be less than it should be; the water remaining in the crucible as part of the hydrate would be calculated as the mass of the anhydrous solid. Therefore there is a LOWER value calculated for the amount of water in the sample; and HIGHER value for the mass of the anhydrate. 

1. IF you were given that the actual formula of the hydrate was actually CuSO45H2O – what could be the source of your error when conducting this lab?

(DISCLAIMER: This response uses the data from above to explain)

It was determined that the coefficient for water was 6 (not 5) from the following calculation: 

mol H2Omol CuSO4  =  0.05660 mol 0.009712 mol  =  5.78

In order to get this number closer to 5 one way would be to decrease the numerator (mol H2O) to get a smaller value instead of 5.78. However, because the sample was heated and massed multiple times and no significant difference in masses was observed - this most likely is NOT the source of the error. 

Therefore, another way is to increase the denominator (mol CuSO4) to get a smaller value instead of 5.78. This would be that there were actually MORE mol of CuSO4 that should have been recorded. If there was any splattering during heating or even touching of the sample with a stirring rod - that could have removed some CuSO4 to impact the results leading to less moles. 

1. You added water to your desiccant salt once your experiment was concluded. Why? What did the results of that test tell you?

To verify that the substance was indeed a hydrate. If water is added to anhydrous substances, the color returns indicating that the hydrate has been regenerated. True hydrates display this property of reversibility. Other compounds that react when heated to produce water are not hydrates if they are unable to reabsorb the water they have lost.

Possible Sources of Error

Not heating the hydrate enough

• Ratio of anhydrous salt to water will not be accurate, as water will remain in sample 

• Fewer moles of water and more moles of salt will be calculated in the hydrate than reality - should be more water and less salt

Overheating the sample

• Anhydrous salt could decompose in the heat

• It appears that the salt is composed of more water than it is

Salt sticks to spatula or is spilled in the process of the lab (splatters when heated)

• It will appear that there is more water in the sample as actual; more moles of water will be calculated in the sample - since any loss of mas would be calculated as water lost

Crucible is weighed while still warm

• Inaccurate mass will be obtained 

Anhydrous salt exposed to air prior to measurement, absorbed some moisture 

• The salt regained some moisture from the air after heating, during cooling, prior to measuring

• The quantity of water calculated would be LESS than what is actual, if there is water regained by the crystal structure (see first point)

• This error is unlikely however, unless a prolonged set of time was in between heating and weighing. Also the crystal would have turned colors showing it hydrated. 

• Oftentimes this lab recommends the use of a desiccator to help avoid any error related to this point. 

Related Problems/ Previous AP FRQs

Below are Free Response Questions from previous AP®︎ Chemistry Examinations:

*AP®︎ and Advanced Placement Program are registered trademarks of the College Entrance Examination Board (“College Board”), which was not involved in the production of and does not endorse this guide. 

• FRQ 2008 #2

• 2008 Free-Response Questions

• 2008 Scoring Guidelines

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