Overview: The main things I want you to learn from this lab are:
What a hydrated salt is
How to use experimental data to determine the empirical formula of a hydrate
How hydration affects the properties of a given salt
The lab is broken up into 2 procedures.
Procedure A is a chance for you to work with a salt of known formula to
learn how to analyze a hydrate. Procedure
B will ask you to determine the formula of an unknown hydrate.
Data and Calculations: In your Student Lab book, generate a data table, write your calculations during the experiment and answer the questions.
Procedure A: (Using the salt CuSO45 H2O)
1. Calculate the theoretical percentage of water in this hydrated salt from the formula.
2. Get a dry pasteur pipet and weigh it. Record the weight.
3. Add roughly at least half of a gram of the hydrated CuSO4. It is best to weigh the salt directly in the pipet. Try to spread the salt out as much as possible without loosing any from the ends. Record the weight of the pipet + salt.
4. Using your percentage values from step 1, calculate how much of your salt's weight (in grams) is actually CuSO4, and how much is water weight.
5. Place the pipet securely on a clay triangle on a ring attached to a ring stand. Be sure that the pipet’s glass is not over any white ceramic. (The ceramic will insulate the pipet making it harder to dry out the salt sample.)
6. Gently heat the salt in the pipet by holding the bunsen burner flame up to the pipet and gradually heating the entire length of the pipet. You should see signs of water vapor condensing at the edges of the pipet, as well as a distinct change in the appearance of the salt.
7. Continue heating to remove all traces of water from the pipet.
Note: if you heat your tube too strongly, you will see a yellowish color appear in the tube and a smell of sulfur will appear. This is bad. It means you have "overcooked" your salt to the point where the sulfate itself is decomposing. Also be careful not to melt the pasteur pipet, especially the tip.
8. When you think you have gotten all the water out of the salt, let the tube cool and then weigh it. Find the weight of the salt that remains in the tube. If this weight is in agreement with the CuSO4 weight you calculated in step 4, then you can be satisfied you have done the experiment correctly. If the weights do not agree, consult your instructor.
9. Compare the appearance of your dehydrated CuSO4 to that of the original hydrated salt. The dehydrated salt is called "anhydrous". Hypothesize what will happen if you add some distilled water to the anhydrous salt.
10. Add a few drops of water from a distilled water bottle to your cooled anhydrous CuSO4 in the pipet. Carefully note any changes in the temperature of the mixture. You may add more water if you wish to see what will happen.
11. When you are finished, empty out the pipet and throw it away in the glass-containing box.
Procedure B: (to be performed on an unknown salt)
In this procedure, you will not have the exact formula of the salt you are working with. You will know what salt it is, but not how many water molecules are "attached" to it. Your task is to determine by experiment how many H2O's belong in the formula, i.e. the empirical formula of the hydrate. The number of H2O's in a hydrate is almost always a whole number. The number can be as small as 1 H2O and as high as 18 H2O.
Your procedure will involve taking a sample of one of the unknown salts, heating it to drive out the water, and seeing how much of its mass is lost. From the mass lost, you should be able to determine empirical formula of the hydrate. (N.B. Getting the correct value does count so work carefully but efficiently.)
Questions:
1. In Procedure A compare your experimental mass loss to your calculated mass loss as a percent error.
2. What experimental “mishap” may explain the fact that a lab group’s value of the mass of water is greater than it should be?
3. Calculate the empirical formula of your unknown hydrate in Procedure B and write your final formula. (Clearly show your work for partial credit.)
Write-up: Please submit your results and summary of the lab on the carbon copy pages of your Student Laboratory Notebook with the following contents:
Data Table: Consolidate all of your data in a single table (no calculations please). Include data from Procedure A and B. After this, record your observations. Include here your hypothesis and your observations to support of deny your hypothesis.
Calculations: Work out with steps calculations for determining the masses of the hydrated salts, dehydrated salts and water lost. Be sure to label your work clearly. Don’t forget to include here your calculations of percent composition and predicted masses.
Questions: Answer the questions as instructed following this work. For questions involving calculations be sure to show your steps clearly and neatly.
