Expt 041 -- Formula of a Hydrate

Mass relationships are at the heart of chemistry. A known mass of copper sulfate pentahydrate is heated to produce anhydrous copper sulfate. The mass of CuSO₄ found is compared to the amount predicted.

Safety

Hot glass and hot objects can cause burns. Hot glass and cold glass look exactly alike. Hold a finger near a previously-heated object for several seconds before touching it to detect heat. Do not grasp hot objects. The last manipulative step where water is added is dangerous; handle with care to avoid burns. Do not place hot objects on balance pans. Wear goggles and apron. Know the location of the fire equipment; review use of that equipment before beginning work. Wash hands after the experiment.

Procedure

1. Obtain a glass Pasteur pipet with a short tip. set up a ring stand with a ring and Nichrome gauze for heating. The gauze should be about 3 cm above the end of the barrel of a bunsen burner. Using a hot bunsen burner flame, heat the empty pipet to drive off water. Heat the tip for brief periods only. The tip will bend and seal if it is heated too long. Sweep the flame from the large end to the tapered end. Repeat several times. Cool to room temperature.

   !!!Click here to See Movie.

2. Determine the mass of the pipet to the nearest 0.001 g.

   !!!Click here to See Movie.

3. Preweigh a sample of 0.45-0.55 g of copper sulfate pentahydrate on a balance paper. Using a small spatula, transfer the copper sulfate pentahydrate into the pipet through its large open end.

   !!!Click here to See Movie.

4. Determine the mass of the pipet plus copper sulfate pentahydrate to the nearest 0.001 g.

   !!!Click here to See Picture.

5. Using a non-luminous flame, heat the copper sulfate pentahydrate. Begin at the open end, and heat slowly and carefully moving the burner toward the tapered end.

   !!!Click here to See Movie.

6. When heated, the blue copper sulfate pentahydrate is transformed to white copper sulfate. When the entire sample has turned white, sweep out the remaining water by moving the flame from the large end of the tube to the tapered end. Repeat until no water is observed.

   !!!Click here to See Movie.

7. Turn off the burner and allow the pipet to cool to room temperature.
8. Determine the mass of the pipet plus copper sulfate to the nearest 0.001 g.

   !!!Click here to See Picture.

9. Repeat the heating and cooling steps until a constant weight is obtained.
10. Place room temperature pipet of white solid in a beaker with the tapered end down. The pipet may become too uncomfortable to hold. Using a wash bottle and holding the pipet at the top where there is no solid, add a drop or two of water to the contents of the pipet. Note and record any evidence for reaction.

    !!!Click here to See Movie.

    !!!Click here to See Picture.

11. Carefully bring a finger near the moistened solid to see if there is a temperature change. Note and record the result.

Data Analysis

1. Predict the mass of copper sulfate expected to result from complete reaction of the copper sulfate pentahydrate.
2. Compare the mass obtained from that predicted.
3. Discuss the reason for any discrepancy.

Questions

1. Write a balanced equation for the copper sulfate pentahydrate decomposition reaction.
2. Predict the mass of copper sulfate expected from heating 0.500 g copper sulfate pentahydrate.
3. Suppose the mass actually obtained after one heating was 0.333 g. Suggest a possible reason for the problem.
4. Anhydrous copper sulfate is used as a dehydrating agent. Based upon the observations, comment on this usage.

Handout Makeup

Name ___________________________ Class _______

Teacher __________________________

SmallScale 041 Formula of a Hydrate

Watch the movies.

Use this sample data for the calculations and to answer the questions.

mass heated pipet - 2.871 g

mass pipet + CuSO₄•5H₂O - 3.340 g

mass pipet + CuSO₄ 1st - 3.171 g

mass pipet + CuSO₄ 2nd - 3.171 g

Answer the questions.

Curriculum-

This activity fits in as soon as mass relations are studied. Excellent quantitative results are expected. Related experiments include decomposition of malachite (SSExperiment 042) and the conversion of NaHCO₃ into Na₂CO₃ (SSExperiment 043).

Safety-

• Hot glass and hot objects can cause burns. Hot glass and cold glass look exactly alike. Hold a finger near a previously-heated object for several seconds before touching it to detect heat. Do not grasp hot objects. The last manipulative step where water is added is dangerous; handle with care to avoid burns. Do not place hot objects on balance pans. Wear goggles and apron. Know the location of the fire equipment; review use of that equipment before beginning work. Wash hands after the experiment.
• Burns are the biggest problem, possibly followed by cuts from broken tubes. When the period begins, remind students to test previously heated objects for heat before grasping them. The rehydration releases a great deal of heat, enough to cause a burn. Advise students to follow the handling instructions. A 500-1000 mL jar is a good choice fo the rehyration so that, if a pipet is released into the jar, it can be supported.

Time-

Teacher Preparation: 10 minutes

Class Time: 45 minutes (Cooling periods may be used to discuss data analysis or other topics.)

Materials-

• copper sulfate pentahydrate, CuSO₄•5H₂O, 0.6 g/reaction.
• Pasteur pipet with short stem
• top-loading balance sensitive to 0.001 g.
• weighing paper
• wash bottle with distilled water
• small spatula
• burner, ring, ringstand, Nichrome gauze, ignitor or matches

Disposal-

Dissolve the solid in water and discard at the sink with large volumes of water.

Lab Hints-

• Warn students to heat the tips cautiously and slowly. The tips will bend and seal shut easily.

  !!!Click here to See Picture.

• Although balances sensitive to the nearest milligram are desired, this experiment gives adequate results when only centigram balances (nearest 0.01 g) are available.

Data Table-

mass heated pipet- 2.871 g

mass pipet + CuSO₄•5H₂O- 3.340 g

mass pipet + CuSO₄ 1st- 3.171 g

mass pipet + CuSO₄ 2nd- 3.171 g

Data Analysis-

The mass of CuSO₄ found, 0.300, is precisely the amount predicted.

Answers-

Q1. Write a balanced equation for the copper sulfate pentahydrate decomposition reaction.

A1. CuSO₄•5H₂O --> CuSO₄ + 5 H₂O

Q2. Predict the mass of copper sulfate expected from heating 0.500 g copper sulfate pentahydrate.

Q3. Suppose the mass actually obtained after one heating was 0.333 g. Suggest a possible reason for the problem.

A3. The reaction probably was not complete. The tube should be reheated repeatedly until it reaches constant weight.

Q4. Anhydrous copper sulfate is used as a dehydrating agent. Based upon the observations, comment on this usage.

A4. The energy release is so large that CuSO₄ is expected to be an excellent dehydrating agent.

CoopLearn-

The data from many groups should be gathered and analyzed to get a good impression of the precision of the experiment. Assign some groups the related experiments 42 and 43.

Key Words 1-

stoichiometry, mass relations, decomposition reaction, water of hydration, balance

Elements-

Cu S