Determining the Formula of a Compound

The technique used in this experiment is called a precipitation titration. A precipitate is the solid formed when two solutions are mixed. In this lab, Co⁺² from a cobalt (II) chloride solution will react in a certain ratio with the OH^-1 (hydroxide ions) from a sodium hydroxide solution. This will form a solid, or precipitate. The sodium and chloride ions will stay in solution and not participate in the reaction. When all of the Co⁺² have been used to make the precipitate, a color change will take place. An indicator, phenolphthalein, is added to react with the OH^-1 once the Co⁺² are used up. Therefore the color change will show the end point of the titration.

The purpose of this lab is to determine the formula of the solid compound formed in the reaction of cobalt (II) chloride with sodium hydroxide.

Lab Writeup

1. Introduction/Background:Follow the lab format handout.
2. Hypothesis: State what you think the formula will be and why.
3. Procedure: Copy this into your lab book.
   1. Materials: Sodium hydroxide solution, cobalt (II) chloride solution, 2 single row microplates, phenolphthalein solution, micropipets, and goggles.
   2. Procedure:
      1. In one of the microplates, add 1 drop CoCl₂ solution to the 1st well, 2 drops to the 2nd well, continuing until there are 11 drops in well #11. The last well is empty.
      2. Add 1 drop phenolphthalein solution to each well.
      3. Add 11 drops NaOH solution to the 1st well, 10 to the 2nd well, continuing until there is 1 drop in well #11.
      4. Use a plastic toothpick to mix each well. Wipe the toothpick with a tissue after each well so you do not contaminate the next well.
      5. Repeat steps 1-4 with the second microplate.
      6. Record the number of drops of CoCl₂ solution and NaOH solution in each well for both trials.
      7. Observe and record the color of each well as well as the level of the precipitate.
      8. Note which well is the first one to remain magenta (dark pink). The wells after it will be magenta, but note the first one.
      9. Clean the microplates by rinsing several times with water. Any residual stain can be wiped out with a cotton swab.

 

4. Data: Your data table for each trial could look like:

5. Data Analysis:

1. For each trial, calculate the ratio of the drops of OH^-1 needed to react with Co⁺².
   Ratio = (OH^-1 drops)/(Co⁺² drops)
2. Explain the meaning of the drop ratio calculated in step A.
3. Write the two formulas of cobalt hydroxide using the ratios calculated in step A.
4. Look up the possible charges for cobalt. Are your formulas possible?
5. If you could choose only one trial, which would you choose? Why?
6. After you added the required drops of Co⁺² and OH^-1 to the wells, did you notice any other colors in the wells? What were they? How do you account for these colors?
7. Cobalt also has a +3 charge. If you had used it instead of the +2, how would the results have differed? Give an example to explain.

6. Evaluation: as usual.

7. Conclusion: as usual.

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