Analía Bellizzi – Chemistry Classes

Ronald Reagan Senior High School

# Percent Composition of a Hydrate

Introduction

In this experiment, you will be heating a hydrate of copper (II) sulfate (CuSO4•nH2O) to evaporate the water. Masses are measured before heating to determine the mass of the original sample (the hydrate) and after heating to determine the mass of copper (II) sulfate (CuSO4) anhydrous. The difference between these two masses is equal to the mass of the water lost. Heating time and temperature are critically important for this experiment. If not enough heat is applied, some water will remain attached to the copper sulfate producing a low calculated mass percent water for the hydrate.

The pentahydrate is 100% isolable only in temperatures lower than 30 °C. The light blue trihydrate non-isolable form can be obtained around 30°C. White monohydrate form is available at 110°C, while the anhydrous form can be isolated near 250°C. If too much heat is applied, the anhydrous copper (II) sulfate (CuSO4), which has a grayish white color, decomposition starts at 250 degrees, while complete decomposition occurs around 600 degrees…

The reaction for the decomposition is as follows:

CuSO4 •5H2O(s)==> SO2(g) + CuO(s)+ 5H2O

Purpose:

1. Determine the percent of water present in a hydrated copper (II) sulfate (CuSO4 • n H2O) and Epson Salts
2. Calculate the value of  “n”, the number of  moles of water molecules present per mole of CuSO and Epson Salts

Materials: – for Part I and Part II

• Hot plate
• glass beaker
• glass rod
• beaker tongs
• Balance (at least 2 decimals)
• Copper sulfate sample (2.5 to 3.5 g)
• Epson Salt (Magnesium Sulfate Sample (Around 5.0 g)

ProcedurePart I

1. You need to grind the sample of hydrate salt using the mortar and pestel to obtain very small particles (The smaller, the better) .
2. Turn on your hot plate.
3. Be sure your beaker and glass rod does not have any crack, and that that they are clean and dry.
4. Measure the mass of the empty beaker with the glass rod inside.
5.  Record this value in your own data table with the maximum available precision.
6. Transfer about 2.5 to 3.5 g of CuSO4●nH2O in your beaker and weigh it, together with the glass rod. Record this value in your data table with the maximum available precision.
7. Calculate the mass of the CuSO4●nH2O by difference between your second and your first measurement. Record that value in the data table below
8. Place your beaker with the sample and the rod on the hot plate.
9. Heat the compound gently Note the release of any steam from the beaker.
10. Continue heating gently until the salt turns completely light grey. Be careful not to overheat! The heat can become so intense that the sulfate in the salt begins to break down. If this happens the salt will turn yellow and produce a sulfurous smell. It will also ruin your data since it will reduce the mass more than expected due to the decomposition of the salt.
11. Use the glass rod to stir the chemical to avoid overheating in some areas. Use the glass end to stir the compound.
12. Stop heating when the salt has lost all traces of blue color. If the compound turn yellow, remove it from the hot plate and place it on a ceramic pad (DO NOT PLACE THE HOT BEAKER ON THE COOL TABLE, IT CAN SHATTER) Allow the beaker and its contents to cool completely.
13. Once the beaker is cool, measure the mass of the beaker, the anhydrous salt and the glass rod.
14. Heat the contents again for a short time (3 minutes). Then allow it to cool and weigh it. If the mass is the same as the previous weighing, then the salt has been completely dehydrated. If not, repeat the heating/cooling/weighing process until two successive weighing have the same mass.
15. Record the final mass of the anhydrous salt in you lab notebook and do the calculations to show that the molar ratio of water to anhydrous salt really is 5:1.
16. Empty the anhydrous salt onto a large watch glass. Use the dropper to add a very little water to the anhydrous copper (II) sulfate.
17. Describe what happens in your lab notebook. For your report explain what is happening at the molecular level when you add water.
18. When you finished this part of the lab empty all the CuSO4 into the beaker provided by your instructor for this purpose.

 Data Table measurements Mass of  hydrate …………………g Final mass of Anhydrous salt(without water) …………………g Mass of water contained in the hydrate …………………g Mass % of water contained in the sample …………………g # Moles  in the anhydrous mass …………………mol Moles of water (mass of water / 18) …………………mol Ratio between  moles of anhydrous salt /moles of water .…………/……..….. Write down the formula of the hydrate  →

Observations Part 1

1. Describe the Copper (II) sulfate hydrate before heating
2. Describe the changes you can observe during the heating process.
3. Describe the way the anhydrous compound looks like.

Observations Part 2

1. Describe the magnesium sulfate hydrate before heating
2. Describe the changes you can observe during the heating process.
3. Describe the way the anhydrous compound looks like.

Analysis Questions

1. How many moles of copper (II) sulfate (CuSO4) did you have in your original sample?
2. How many moles of water did you have in your original sample?
3.  Examine the formula for the hydrate: CuSO4 • nH2O. Notice that “n” is “the molar ratio of water to copper sulfate.” Find the numerical value for in this sample (use your numbers from part a above).
4. The actual mass percent of water in the hydrated copper (II) sulfate compound should have been 36.1%. Compare this value to the experimental percentage you obtained.
5. In the experiment involving hydrated copper sulfate, overheating causes a high calculated percent value for water.
1. Why is the high reading obtained? Hint: Overheating causes copper (II) sulfate, CuSO4 to turn into copper (II) sulfide, CuS.
2. What is lost from the CuSO4 in this process?
6. Where does it go?
7. What experimental evidence would you have to indicate you inadvertently overheated the hydrated copper sulfate compound? Hint: Re-read the “Experimental Considerations” section.
8. Determine the mass percent of each element present in CaCO3.
9. Determine the number of grams of Na present in 4.0g of NaOH.
10. Calculate the mass percent water in MgSO4 • 2 H2O

#### Your LAB REPORT should include

• TITLE
• PURPOSE
• MATERIALS
• PROCEDURES
• LAB SETUP
• DATA TABLE
• CALCULATIONS (NO CALCULATIONS WILL RESULT IN AN “I” (INCOMPLETE)
• ANALYSIS QUESTIONS