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Formula of an Unknown Hydrate
TABLE OF CONTENTS:
Lab Handouts
Student Lab Handout: Document contains basic lab procedure and purpose. Standard for student use.
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Annotated Lab Handout: Lab handout above with basic procedure steps in addition to notes/ annotations that explains the use/meaning behind each step of the procedure. In addition, helpful notes that could be useful to the teacher/ instructor while conducting the lab.
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Data Table: Printable data table to record collected data from the lab
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Procedure Summary
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
Related concepts include: Compounds vs Mixtures, Hydrate, Law of Definite Proportions, Chemical formulas
Chemical/ Material Preparation Notes
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Copper(II) sulfate hydrate
Deionized water
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Balance (analytical)
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Bunsen burner
Crucible with lid
Desiccator (optional)
Crucible Tongs
Ring stand
Clay Triangle
Iron ring
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All liquids and solids can be disposed of in the heavy metal inorganic waste container for proper disposal.
Before the start of the lab, be sure to inspect all crucibles to be sure they are in good condition. If a crack is noticeable in a crucible it should be discarded to avoid it from shattering when heated strongly.
Optional Procedure Modifications
This section details optional variations of the lab to accommodate different learning styles, resource availability, and curriculum goals.
The following are possible other hydrates that can be used in this experiment:
$\text{BaCl}_2 \cdot 2\text{H}_2\text{O}$
$\text{MgSO}_4 \cdot 7\text{H}_2\text{O}$
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!) Teachers may not use crucible lids because students have a difficult time using tongs to partially cover without the lid accidentally falling off and shattering.
Microscale or Low Cost Alternatives
Instead of using a crucible, a small amount of the hydrate (0.1-0.2 g) can be heated in a test tube held horizontally with a clamp. The mass can be measured using a centigram balance if an analytical balance is not available. The water vapor will condense at the cooler, upper end of the test tube. This version reduces chemical waste and can be performed with less specialized equipment.
Inquiry-Based or Open Ended Design Versions
Instead of giving students a specific hydrate, provide them with several different, unlabeled hydrates. Students must first qualitatively test the hydrates to observe their properties (e.g., color change upon heating) and then design an experiment to determine the empirical formula of one of them. Students would need to decide on the amount of sample to use and the heating time.
⇒ This version promotes critical thinking, experimental design skills, and problem-solving.
Remote/ Virtual Adaptations
At Home Version: If students have access to a kitchen scale, they can perform a similar experiment with Epsom salt (magnesium sulfate heptahydrate, MgSO4·7H2O), which is readily available. The Epsom salt can be heated in an oven-safe dish at a low temperature (e.g., 250°F or 120°C) for an extended period to drive off the water. While the results will not be as precise as with lab equipment, the conceptual process is the same.
⇒ This provides a hands-on experience for remote learners, connecting chemistry concepts to everyday materials.
Pivot Interactives (Virtual): Pivot Interactives has a video-based experiment on determining the formula of a hydrate. Students analyze a video of the experiment being performed and collect data from the video to perform the calculations. [External Link]
Also check out the section Links to Similar or Related Labs.
Vocabulary/ Important Terms
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
Pre-Lab Question Bank (with answers)
Below are some standard questions that students can complete to prepare for the lab work:
Question(s) related to: Related Chemical Concepts
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.
| Hydrate | Chemical Formula | Formula Mass | % Mass of Water |
|---|---|---|---|
| nickel(II) chloride hexahydrate | |||
| cobalt(II) chloride hexahydrate | |||
| copper(II) sulfate pentahydrate |
Solution to Question Above:
1. See table below
| Hydrate | Chemical Formula | Formula Mass | % Mass of Water |
|---|---|---|---|
| nickel(II) chloride hexahydrate | NiCl₂·6H₂O | 237.69 g/mol | 7.58% |
| cobalt(II) chloride hexahydrate | CoCl₂·6H₂O | 237.93 g/mo | 7.57% |
| copper(II) sulfate pentahydrate | CuSO₄·5H₂O | 249.68 g/mol | 7.21% |
Question(s) related to: Sample Data
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.
(a). Mass of hydrate
(b). Mass of anhydrous CoCl2
(c). Mass of water driven off
(d). Moles of water
(e). Moles of anhydrous CoCl2
(f). Moles of water per mole of CoCl2
(g). Formula of hydrate
Solution to Questions Above:
(a) 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
(b) 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
(c) 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
(d). 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
(e) 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
(f) 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
(g) 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
Define a hydrate.
Describe how to handle the crucible during the lab investigation and explain why.
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?
How can you tell visually that the water escaped from the hydrate?
How many times does the hydrate need to be heated?
Solutions to Questions Above
Hydrates are compounds that have one or more water molecules integrated into their structure.
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.
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.
The appearance of the solid changes color from blue to white.
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
| Mass of empty crucible (g) | 22.68 |
| Mass of crucible + hydrated sample (g) | 25.25 |
| Mass crucible + sample (after first heating) (g) | 24.23 |
| Mass crucible + sample (after second heating) (g) | 24.22 |
Possible Observations students may make:
The initial hydrate sample is a crystalline, blue solid.
During heating, condensation (water) is observed on the inside of the cover and at the top of the crucible.
The color of the solid changes from blue to a pale blue-white as it is heated.
The final anhydrous solid is a fine, white-gray powder.
Sample Analysis/ Calculations
There are several hydrates that can be used to conduct this lab. For this version, the hydrate in question is:
$$\text{CuSO}_4 \cdot x\text{H}_2\text{O} \longrightarrow \text{CuSO}_4 + x \text{H}_2\text{O}$$
Question 1: Mass of hydrated sample = (Mass of crucible + sample) - (Mass of empty crucible)
= 25.25 - 22.68
= 2.57 grams
Question 2: 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 $\text{CuSO}_4 \cdot {x}\text{H}_2\text{O}$ used in this lab.
Question 3: 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 $\text{CuSO}_4$ in the sample of $\text{CuSO}_4 \cdot {x}\text{H}_2\text{O}$ used in this lab.
Question 4: Convert water to moles:
1.02 H2O x 1 mol H2O18.02 g H2O = 0.05660 g H2O
Question 5: Convert anhydrous salt to moles:
1.55 g CuSO4 x 1 mol CuSO4159.60 g CuSO4 = 0.009712g g $\text{CuSO}_4$
Question 6: Calculate ratio/ Formula of the hydrate:
mol H2O / mol $\text{CuSO}_4$ = 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: $$\text{CuSO}_4 \cdot 6\text{H}_2\text{O}$$ (copper(II) sulfate hexahydrate)
Discussion Questions
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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.
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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.
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(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 H2O / mol 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.
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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.
Links to Similar Labs
This section provides a list of other labs that are related to the concepts or procedures in the hydrate lab.
Alternatives
The following are very similar hydrate labs from different sources and slight modification approaches to this lab.
Other examples of sample calculations using hydrate word problems: Unknown Hydrate Determination: Chemistry Sample Problem
Conceptually Related Labs
Lab Title: Empirical Formula of Magnesium Oxide
Brief Description: Students burn a known mass of magnesium in the presence of oxygen to form magnesium oxide. By measuring the mass of the magnesium and the magnesium oxide product, they can determine the empirical formula of the oxide.
Connection Type: Conceptual. This lab also involves determining an empirical formula from mass data and reinforces mole ratio calculations.
Lab Title: Percent Composition of a Mixture
Brief Description: Students are given a mixture of two salts (e.g., sodium chloride and sodium carbonate) and must determine the percent composition of the mixture by mass. This can be done by selectively reacting one of the components and measuring the mass change.
Connection Type: Conceptual. This lab also uses gravimetric analysis and stoichiometry to determine the composition of a substance.
Procedurally Related Labs
Lab Title: Gravimetric Analysis of a Chloride Salt
Brief Description: Students dissolve a known mass of an unknown chloride salt in water and then add an excess of silver nitrate solution to precipitate all of the chloride ions as silver chloride. The mass of the dried silver chloride precipitate is then used to calculate the percentage of chloride in the original salt.
Connection Type: Procedural. This lab uses the same core techniques of gravimetric analysis, including careful mass measurements, precipitation, and heating to a constant mass.
Lab Title: Decomposition of Sodium Bicarbonate
Brief Description: Students heat a sample of sodium bicarbonate (baking soda) and measure the mass loss to determine the stoichiometry of the decomposition reaction. The products are sodium carbonate, water, and carbon dioxide.
Connection Type: Procedural. This lab also involves heating a compound to cause a decomposition reaction and using mass loss to analyze the reaction.
Also check out Possible Procedure Modifications section.
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
This question specifically addresses the determination of the formula of a hydrate (MgCl₂·xH₂O) through experimental data, including heating to a constant mass.
Conceptually Related
FRQ 2014 #1
This question involves stoichiometry and gravimetric analysis, which are key skills used in the hydrate lab.
FRQ 2011 #2
This question involves stoichiometry and gravimetric analysis, which are key skills used in the hydrate lab.
GoogleSheets Data & Analysis Tool
GoogleSheet Data & Analysis Tool helps to record data and provides real time feedback. Once all the data is recorded, the full analysis and calculations are completed instantly allowing the experimenter to validate results and focus on lab technique.