Many of you had the opportunity in the first semester General Chemistry Laboratory to analyze an unknown containing metal ions of the acid chloride group. These ions, Ag⁺, Hg₂²⁺, and Pb²⁺, all formed precipitates in the presence of hydrochloric acid. In this experiment, you will be asked to identify four other metal ions: Ba²⁺, Cu²⁺, Ni²⁺, and Fe³⁺. Our strategy will be to precipitate the barium ion as its sulfate salt, while the other ions will be precipitated as hydroxides and then treated with reagents that will allow the identification of each ion in turn.

While completing this experiment, you will be expected to apply your observation skills and deductive reasoning to determine the composition of an unknown. To aid your analysis, you will first conduct the qualitative tests on a solution containing all five possible ions. The flow diagram for the analysis is presented in Figure 1.

1. Write the net ionic equations for the following reactions:
1. Barium chloride with sulfuric acid
2. Iron(III) nitrate with sodium hydroxide
3. Copper(II) ion with iron metal
2. In a suitable reference, such as the CRC Handbook of Chemistry and Physics, look up and record the color of the following: barium sulfate, copper(II) hydroxide, nickel(II) hydroxide, iron(III) hydroxide, and nickel(II) dimethylglyoxime. Based on your observations made during Experiment 4 (A Study of Chemical Equilibrium), what will be the color of FeSCN^-? What is the K_sp at 25 °C for barium sulfate, copper(II) hydroxide, nickel(II) hydroxide, and iron(III) hydroxide?
3. If you have a solution that contains 0.05 M Ni(NO₃)₂, what must the concentration of NaOH be to just begin precipitation of Ni(OH)₂? Use the relevant K_sp that you identified in question 2 to solve this problem.

Be sure to take careful notes on color changes; the formation or dissolution of precipitates; the color, quantity, and type of precipitates. Mix the test tube contents well after the addition of each reagent. To test a solution for acidity or basicity, add a drop of acid or base to the test solution, stir the mixture, and touch the stirring rod to a piece of moistened litmus paper that has been draped over a watch glass.

Place 5 drops of 0.2 M BaCl₂, 5 drops of 0.2 M Cu(NO₃)₂, 5 drops of 0.2 M Ni(NO₃)₂, and 5 drops of 0.2 M Fe(NO₃)₃ together in a single test tube.

Add 6 drops of dilute (3 M) H₂SO₄ to the sample just prepared containing all the metal ions in the test tube and mix well. Centrifuge the tube. Do you see a precipitate? (We’ll call this Precipitate 1.) Record its appearance. Add 2 more drops of H₂SO₄ to test for complete precipitation. If more precipitate forms add another drop and continue until no more precipitate is seen to form. Transfer the liquid portion (Supernatant 1) to a new test tube and for analysis of Cu²⁺, Ni²⁺, and Fe³⁺.

Add 2 M NaOH to Supernatant 1 drop by drop until the solution is just basic. Centrifuge the test tube. Add 1 more drop of 2 M NaOH to test for complete precipitation. Continue adding NaOH, centrifuging after each drop, until precipitation is complete. Discard the liquid portion. Note the appearance of the precipitate (Precipitate 2).

In the hood, add 1 mL of H₂O and 1 mL of 6 M NH₄OH to Precipitate 2 and stir the mixture. Note the appearance of the solution and remaining Fe(OH)₃ precipitate. Centrifuge the test tube. Save the precipitate (Precipitate 3) for the Fe³⁺ test. Split the resulting liquid portion (Supernatant 2) into two tubes. To one tube add 3 M CH₃CO₂H, dropwise, until the solution is nearly colorless. Add a few grains of NaCl. Gently sand an iron nail and place it in the solution. Leave the nail in the solution, undisturbed, for 15-30 min. Record your observations.

To the second half of Supernatant 2, add 3 M CH₃CO₂H, dropwise, until the solution is nearly colorless. Then add a few grains of dimethylglyoxime (DMG). Note the appearance of the mixture.

Test for Fe³⁺

Obtain an unknown from your instructor, record its number, and repeat the procedure above. A positive test indicates the presence of that ion in the unknown. Each unknown contains at least two metal ions.

The assignment for this experiment is to complete the flow diagram for your unknown solution using the accompanying worksheet. Note that only the ions you positively identified in your sample should be written in the boxes along with your observation. For example, if copper metal did not plate out on to the iron nickel, then write "No Cu" in that box. The acidified solution that the iron nail was added to, therefore, did not include Cu(NH₃)₄²⁺ so that would be removed from the box above and so on to the top box. In this way the contents of your unknown solution will be written in the first box of the flow diagram.

Under the flow diagram, please complete the statement "Unknown #____ contained the following ions: ______________." Each unknown has at least 2 different metal ions.