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Formula of a Hydrate Lab

By Caroline Duesing
Last updated about 4 years ago
22 Questions
Note from the author:
Formula of a Hydrate Virtual Lab
Follow the link below to access the virtual lab. You will need to enable flash on your device.


http://introchem.chem.okstate.edu/DCICLA/Empirical.html
Background: Hydrates are ionic compounds (salts) that take up specific amounts of water molecules as part of their crystal structure. This water can be driven off by the application of heat (vaporized).
Table salt usually contains a small amount of hydrated calcium chloride, CaCl2 • H2O, and a hydrated magnesium chloride, MgCl2•6H2O. On humid or rainy days these two impurities absorb moisture and cause the table salt to clump together, making it hard to pour. To prevent this, Morton Salt added an anti-caking agent so “when it rains, it [still] pours”.

Purpose: In this lab you will calculate the percent composition of water in a hydrate and determine the empirical formula of the hydrate you are working with.

Pre-Lab Questions:
1.

What two things make up hydrates?

2.

In order to determine the percent composition and the empirical formula of a hydrate, you must know how much water is in the hydrate. As you cannot measure the mass of the water as it gets added to the salt, how can you determine this?

3.

What is the formula for copper(II) sulfate?

4.

What is the molar mass for anhydrous (without water) copper(II) sulfate?

Materials: Bunsen burner, ring stand, iron ring, clay triangle, crucible, crucible tongs, Copper(II) Sulfate Hydrate, analytical balance, stirring rod
5.

Lab Set Up Diagram: Please draw a picture of the lab set up for this lab

Procedure:

If we were doing the lab in person, we would need to prepare the crucible. Please read through the "Crucible Preparation procedure" below. You will not need to do this part, but there are post lab questions that relate to this process.

Crucible Preparation: The final results of this lab are dependent on getting accurate masses of the hydrate before and after it is dehydrated. In an analytical investigation that involves crucibles, the crucibles must first be cleaned from impurities. Even oils from your fingers as you touch the crucible can cause inaccurate masses. The typical process to clean a crucible is to heat it to constant mass, during which the impurities are burned away.

1. Set up the ring stand with ring clamp, clay triangle, crucible, and burner.
2. Adjust the height of the ring so it will be only a few inches above the top of the burner. With the burner off to the side, light the burner and adjust it to a proper heating flame. (A yellow-orange flame will produce soot on the crucible and counter the process of burning away the impurities.)
3. Place the burner under the crucible, lining up the inner cone with the bottom of the crucible and heat it vigorously for 5 minutes.
4. Turn off the burner and allow the crucible to cool. Only use the crucible tongs to touch/move the crucible from this step forward.
5. Once the crucible has cooled enough that no heat is detected with the back of your hand, carefully carry it to the analytical balance and find the mass of the empty crucible.
6. Repeat the heating and massing steps with the crucible until the masses of two successive heatings match.

Simulation Procedure:
1. Record the initial mass of the hydrate in the data table below.
2. Start heating the sample
3. Record the final mass of the dehydated sample in teh data table below.

Data Table:
6.

Initial mass of the hydrate

7.

Final mass of the hydrate

8.

Observations during the experiment:

Calculations:
9.

Moles of dehydrate: (please show your work below and box your answer)

10.

Mass of water lost: (please show your work below and box your answer)

11.

Moles of water lost: (please show your work below and box your answer)

12.

Experimental % by mass of water in hydrate: (please show your work below and box your answer)

13.

Percent error for the lab assuming the actual % by mass to be 36.1%: (please show your work below and box your answer)

Remember that the formula for percent error is ∣actual - theoretical∣/theoretical x 100%

14.

Based on your calculations, the Empirical formula of the hydrate = ___ CuSO4 • ____ H­2O.

(Please show your work below and box your answer)

Post Lab Questions (Please answer in complete sentences):
15.

Explain why the dehydration is only a physical change even though the compound changed from blue to white.

16.

Explain why the dehydration is only a physical change even though the compound changed from blue to white.

17.

Summarize the process of getting a crucible to “constant mass” to know it is free of impurities. (Refer to the "crucible preparation procedure" above).

18.

What would be the necessary process to determine if the hydrate was completely dehydrated if a salt was used that did not change color? (Consider what would be done to determine if the crucible was completely clean of impurities.)

19.

What would have happened to your results if during the dehydration some of the copper(II) sulfate splattered out of the crucible – would your results show more, less, or the same amount of water in the hydrate? Explain.

Please watch the video below to help you write your conclusion in the "Claim, Evidence, Reasoning" (CER) format.
20.

Conclusion:

What is your claim (this is the answer to the question "What is the formula of the hydrate")?

Please state your claim in a complete sentence.

21.

What is your evidence that supports your claim? (please use specific data/calculations from the lab that support your claim.


22.

What is your reasoning that connects your evidence to your claim? Please also address any sources of error that could impact your data and therefore your claim.