Exp 4: The Identification of an Unknown Solution

It is a given that the unknown solution in this experiment contains a cation belonging to the alkaline-earth family and an anion belonging to the halogen family. These compounds are relatively soluble in water, but there are several other salts of the alkaline-earths that vary in solubility. By carrying out ionic reactions between solutions of each of these cations with solutions of selected anions, such as sulfate, you will have a reference with which to compare your unknown solution's reactions with those same anions.

Halides, the anion form of halogens, can be oxidized to the elemental form. In fact, one kind of replacement reaction is that between a halide and a halogen. A halogen will replace a less active halogen (in it's halide form) from a compound. By carrying out a series of replacement reactions between halogens and halides, you can determine the relative oxidizing strengths of the halogens and use this information to identify the unknown halide in your solution.

PROCEDURE

I. Relative Solubilities of Alkaline Earths

Place about 15 drops of 1 M Na₂SO₄ in each of 5 wells in row A of a 24 well plate. Then place the same amount of 1 M Na₂CO₃ in 5 wells of row B, 0.25 M Na₂C₂O₄ in row C, and 0.1 M K₂Cr₂O₇ in row D.

Now add the nitrate salts of magnesium, calcium, strontium, and barium so that each alkaline earth cation is tested with each of the anions from above. Be sure to note how many drops are added to get any precipitates and the appearance of each. Stop adding after 12 drops if there is still no ppt, but check after a few minutes to see if there is any delayed reaction. Leave the last well of each for the unknown.

Sample well plate.

II. Relative Oxidizing Powers of the Halogens

Add a small amount of chlorine water to one of 3 small test tubes and then add 1 mL of cyclohexane. Stopper and shake well. Allow the layers to reappear and note the appearance of the top layer. Repeat with the bromine water and iodine water. These are your color standards for reference. See the standards.

To each of 3 more small test tubes add about 1 mL of bromine water and 1 mL cyclohexane. Then add 1 mL of 0.1 M NaCl to the first test tube, and the same amount of NaBr to the second, and the same amount of NaI to the third. Stopper and shake well. Compare the color of the top layer to that in the reference test tubes from above. Decide if Br₂ is a stronger oxidizing agent than the halogen that was produced.

Repeat the above procedure twice, first using chlorine water and then iodine water. Again, decide whether or not Cl₂ or I₂ are stronger oxidizing agents than the halogen that was produced.

III. Identification of the Unknown Alkaline Earth Halide

Select unknown A, B, or C, and using the solubility tests from Part I and the oxidizing tests from Part II, determine the identity of your unknown alkaline-earth halide.