10 Qualitative Analysis

Background

In experiment 6, you learned how to identify the presence of common ions by conducting chemical tests, a process called qualitative analysis. But how do you test a solution containing mixtures of ions with similar chemical behavior? For example, both calcium and barium ions react with oxalate ions (C2O42-) to form white precipitates. IF you add oxalate ions to an unknown solution and a white precipitate forms, you cannot say conclusively whether the solution contains only calcium ions, only barium ions, or a combination of both. Additional tests are required to identify the unknown solution.

I this experiment you will observe the reactions that occur when four solutions are mixed with each of three different reagents. Using these reactions as a reference, you will develop a systematic set of tests to identify an unknown solution. The solutions and reagents are deliberately unlabeled: knowledge of the chemicals used is not important.  Instead, focus your attention on making careful observations and developing a logical, systematic approach for identifying the unknown solution.

Goals

Equipment

Safety goggles
1 spot plate
1 plastic wash bottle

Materials
Set of solutions: 1, 2, 3, 4 (BaCl2, AgNO3, Pb(NO3) 3, NH4MoO4)
Set of reagents: A, B, C (K2CrO4, HCl, Na2HPO4)
unknown solutions
paper towels
distilled water

Safety

Procedure

  1. Copy Table 10.1 and use to record data.
  2. Clean the spot plate or test tubes with distilled water.  In separate depressions on the spot plate or in separate test tubes, mix solutions 1 through 4, one at a time with each of the reagents.  Use 1-2 drops of each solution. Record results.
  3. Study your results and plan how to identify your unknown using the fewest number of tests.
  4. Obtain an unknown and identify it as a solution 1,2,3, or 4 by testing it with the reagents.  Record the tests done and the results obtained.
  5. Repeat step 3 with a second unknown solution. Record the tests done and the results obtained.
  6. Follow your teacher's instructions for proper disposal of the materials.


Data

Table 10.1 Results of Mixing Solutions and Reagents

Solution 1

Reagent A

Reagent B

Reagent C

Solution 2

 

 

 

Solution 3

 

 

 

Solution 4

 

 

 

Solution 5

 

 

 

Unknown

 

 

 

Unknown

 

 

 

Conclusions

1.  Without knowing the identify of the chemicals in the experiment, you identified an unknown solution. Explain how.
2. Based on your experimental results, how would you distinguish between solutions 1 and 3?  Between solutions 2 and 3?

Extension

1.  Suppose someone has randomly relabeled the solutions used in this experiment as I, II, III, IV and the reagents as X, Y, and Z.  Using the facts that follow, consult Table 10.1 and correctly identify the solutions and reagents by their original designations (1, 2, 3, 4, A, B, C).

When mixed:
I and Y produce a red precipitate.
II and Y produce no precipitate.
II and X produce a yellow precipitate.
III and Z produce  a white precipitate.