The Formula of a Compound

Introduction

The purpose of this experiment is to determine the nature of the solid compound formed in the following reaction:

sodium hydroxide(aq) + cobalt chloride(aq) --> a precipitate + a solution

Once the formula of the precipitate has been determined, you will be able to establish a balanced equation for this reaction.

The technique used in this experiment is called a precipitate titration. Cobalt ions, Co²⁺, from the cobalt chloride solution will react in a set ratio with the hydroxide ions, OH^-, from the sodium hydroxide solution. This combination will form a precipitate of the compound in question. The sodium and chloride ions will stay in solution. An indicator is added which will change color when the OH^- ions are no longer able to react with the Co²⁺ ions. This will occur when all the cobalt ions have been consumed in the formation of the precipitate. The color change is called the end point of the titration.

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1. Set up a hot water bath
2. Heat the pipets containing the cobalt chloride and sodium hydroxide solutions in the water bath for 15-20 seconds. (Holding the dropper ends with forceps will prevent burns.)
3. Select a 12-well strip. Add 1 drop of 0.1 M CoCl₂ to well 1, 2 drops to well 2, etc., until you add 8 drops to well 8.
4. Add 8 drops of 0.1 M NaOH to well 1, 7 drops to well 2, etc., until you add 1 drop to well 8.
5. Add 1 drop of 1% phenolphthalein solution to each well.
6. Stir each well with a toothpick.
7. Permit the mixtures to settle for 10 minutes. Select a second 12-well strip. Use a Beral pipet to remove a small amount of the liquid from above the precipitate in the first well and transfer that liquid to the corresponding well in the fresh strip. Rinse the transferring Beral with water. Repeat this process until a sample has been obtained for each well.
8. Add 2 drops of NH₄SCN to each of these wells containing the transferred liquid.
9. Test NaOH with both phenolphthalein and NH₄SCN. Test CoCl₂ with both phenolphthalein and NH₄SCN. (Use unoccupied wells on the 12-well strips.)
10. Repeat the procedure if time permits.
11. Save the cobalt residues for disposal with heavy metal wastes. Clean your equipment. Wash your hands.
    

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1. Having experimentally determined the formula for cobalt hydroxide, is this formula reasonable considering the charge balance of the compound. Support your answer.
2. Using the formula for the precipitate product you established, complete the original equation for the reaction. Use chemical symbols in place of words.
3. State the meaning and importance of the results obtained from the test for the presence of cobalt ions in the reacted solution.
4. If you could use only one of the trials, which would you choose? Why?

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1. Determine the ratio of the drops of Co²⁺ needed to react with the drops of OH^- to form cobalt hydroxide precipitate.
2. Assuming that the concentrations of the two reacting solutions are the same, explain the meaning of the drop ratio calculated in question #1.
3. From your answers to questions 1 and 2, what would be the ratio of Co²⁺ ions to OH^- ions in the precipitate, cobalt hydroxide? (Round off your answer to the nearest whole number.)
4. Write the formula for cobalt hydroxide which you have determined in this experiment.
   

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• Wear aprons and eye protection at all times during this experiment.
• Use extreme caution with sodium hydroxide solutions. Sodium hydroxide is caustic to tissue and causes blindness. Wash spills immediately. Cobalt compounds are toxic. Do not ingest any chemicals. Wash spills immediately with water. Wash hands before leaving the laboratory.
• Rinse solutions into a container for heavy metal wastes for proper disposal.

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The experimental ratio is usually 2 OH^- to 1 Co²⁺.

This means that there are 2 moles of hydroxide for every
1 mole of cobalt(II) ions.

This results from the 2:1 ratio expected, or Co(OH)₂.

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Collect all solutions containing heavy metals in a container for proper disposal.

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• This experiment should be done early in a first year course. It illustrates the concept of formula writing and equation balancing.
• This procedure can be completed in the typical one hour period. This includes: (a) pre-lab introduction of purpose, technique, and safety procedures; (b) two complete attempts at the procedure; and (c) post-lab discussion of the concepts involved.
• Drop sizes must be uniform in this experiment. See the special technique in which a thin stem pipet and a plastic pipet tip are combined to make a device that delivers uniform drops.
  

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• 0.10 M cobalt chloride (dissolve 2.38 g of CoCl₂•6H₂O in enough water to make 100 mL of solution)(Note: cobalt compounds present special safety problems; use in other than highly controlled situations is not advised.)
• 0.10 M sodium hydroxide - NaOH (dissolve 0.40 g of NaOH in enough water to make 100 mL of solution)
• 1% phenolphthalein (dissolve 1 g phenolphthalein in 60 mL of 95 % ethanol and dilute to 100 mL with distilled water)
• ammonium thiocyanate reagent (dissolve 5 g of NH₄SCN in 100 mL acetone; store in glass, not in plastic. Do not store this solution; make fresh every few days.)
• 2 12-well strips
• 5 toothpicks
• water bath (250-mL beaker with 200 mL tap water; hot plate)

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The Woodrow Wilson microscale version of this lesson was developed by:

Lee Daniel
Arapahoe High School
2201 E Dry Creek Road
Littleton, CO 80122

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