EXPERIMENT Identification of Unknown Solutions PURPOSE As you work through this exercise you will learn how to: Make careful observations of chemical reactions. Develop logical testing procedures for performing qualitative analyses. Describe solubility rules and fundamental acid/base chemistry. MATERIALS AND EQUIPMENT In addition to the solutions themselves, you may also use the following materials: Universal ph indicator paper Spot-plate Pasteur pipets + bulb DISCUSSION Note: Review solubility rules and net ionic equations in your textbook One of the central roles of a chemist involves the analysis of materials. In some instances the analysis is quantitative, that is, a determination is made of how much of a particular substance is present. In other cases the analysis is qualitative and a determination of what components are present is made. In this experiment you will perform a qualitative analysis of a set of unknown solutions. Your team (four students maximum) must first determine which set of unknowns you have, and then identify what is present in each testtube or bottle. The identification process will involve observations of solution properties such as ph, color, odor, energy change, and the results of mixing two solutions, such as the formation of an insoluble solid (precipitate). The only reagents you will be allowed to use are the unknown solutions themselves. Page 1
A little pre-lab preparation is in order. As an example, suppose you have a set of unknowns consisting of the following solutions (in no particular order): Fe (NO3)2(aq), KOH(aq), Cu(NO3)2(aq), Na3PO4(aq), HCl(aq) By this time you should have covered strong and weak electrolytes in lecture, which include strong and weak acids. Using a universal ph indicator paper you would carry out preliminary tests looking for strong/weak acids and bases. The ph of a solution is a measure of the acidity or basicity of a substance. A neutral solution has a ph of about 7. A solution is acidic if it has a ph below 7 and a solution is considered basic if it has a ph above 7 (see chart below). You may also find a solution weakly acidic or basic respectively if you have a salt that contains a cation which is acidic or an anion which is basic (the conjugate acid of a weak base, or the conjugate base of a weak acid). Additionally, small metal ions (like Al 3+ ) with large charge have acidic properties (Lewis acid/base concept). Having given you the foregoing information let s proceed with how we make use of the information in the example solution set above and their acid-base properties. An estimate of a solution ph can be made the following way. Place a small drop of the solution from your spot-depression plate in contact with universal ph indicator paper and note the color that the wetted paper becomes. Now look for the matching color and associated ph value. We want to note that in this example Cu 2+ ion in solution has a distinctive blue color so the identification of the Cu(NO3)2 solution can be made on the basis of color; the other four solutions are colorless. The unknown solution sets you will be given are all colorless. This example set contains a strong acid, HCl, and a strong base, KOH. A drop of one of the solutions on the ph paper that gives a dark red color, would indicates a ph much less than 7 and a strongly acidic solution, identifying HCl. A testing another solution may reveal a deep blue or purple color and suggest a strongly basic solution that would identify KOH. Testing the remaining two solutions using universal ph indicator paper would suggest which solution is Fe(NO3)2(aq) and which solution is the Na3PO4(aq). As ordered below the first solution contains a Fe 2+ salt and would be neutral. The other solution contains the conjugate base of a Bronsted-Lowry ionic acid, and hence would give a weakly basic solution and must be Na3PO4(aq). [Your instructor will demonstrate this property during the lab lecture.] So in this example you would have been able to identify all of the five solutions on the basis of color and their acidic and basic properties. This will not be the case with some of your unknown solution sets and in order to identify them completely you may want to make observations as to what happens when pairs of solutions are mixed. This requires that you set up a reaction grid as shown below. Page 2
Sol ns Fe(NO 3 ) 2 (aq) KOH(aq) Cu(NO 3 ) 2 (aq) Na 3 PO 4 (aq) HCl(aq) Fe(NO 3 ) 2 (aq) ppt. N.R. ppt. N.R. KOH(aq) ppt. N.R heat evolved Cu(NO 3 ) 2 (aq) ppt. N.R Na 3 PO 4 (aq) heat evolved HCl(aq) By mixing each solution with each of the others and using the solubility rules in Table 1., you can predict the outcome of the reaction and verify what is in each solution. Note that if Cu(NO3)2 solution is mixed with each of the other four solutions, a precipitate of an insoluble copper salt is expected with KOH and Na3PO4, but not with Fe(NO3)2 (see Table 1.). Thus, the identity of the Fe(NO3)2 solution can be made. The possible sets of unknown solutions in this experiment are: Set A: AgNO 3 (aq), Mn(NO 3 ) 2 (aq), Ba(NO 3 ) 2 (aq), HCl(aq), NaOH(aq) Set B: Zn(NO 3 ) 2 (aq), Al(NO 3 ) 3 (aq), AgNO 3 (aq), NaOH(aq), NH 3 (aq) Set C: AgNO 3 (aq), Ba(NO 3 ) 2 (aq), HCl(aq), H 2 SO 4 (aq), NaOH(aq) Set D: AgNO 3 (aq), Pb(NO 3 ) 2 (aq), HCl(aq), NH 3 (aq), H 2 O(aq) As part of your pre-laboratory assignment set up four reaction grids and using Table 1. Predict the results when a small amount of one solution is mixed with each of the others. Also make use of the vast amount of information available to you over the internet and find out what colors the predicted precipitates would be. Table 1. Solubility Guidelines for Salts and Bases in Water Soluble Compounds Compounds containing: NO 3 C 2 H 3 O 2 Cl, Br, or I SO 4 2 Insoluble Compounds Compounds containing: S 2 Important Exceptions None None Compounds of Ag +, Hg 2 2+, and Pb 2+ Compounds of Sr 2+, Ba 2+, Hg 2 2+, and Pb 2+ Important Exceptions Compounds of NH + 4, the alkali metal cations, and Ca 2+, Sr 2+, and Ba 2+ CO 3 2 PO 4 3 OH Compounds of NH 4 + and the alkali metal cations (IA) Compounds of NH 4 + and the alkali metal cations (IA) Compounds of the alkali metal cations (IA), Sr 2+, and Ba 2+, Ca 2+ (slightly soluble) Page 3
PROCEDURE 1. Each team should obtain a set of unknown solutions. Record the unknown number for your set. 2. Carry out preliminary tests to decide which of the four possible sets of unknown solutions you have. Be sure to record detailed observations as you perform these tests (preferrably in your lab notebook). 3. Once you have determined which group of solutions you have, make additional tests (by mixing solutions) so that you can identify what is specifically in each of the five solutions. Caution: When taking samples for testing on the spot-plate, be careful not to contaminate any of the solutions. Use a separate Pasteur or plastic pipet for each solution. When finished turn in your report form (below) and pre-laboratory worksheet for this experiment. Cut along the solid lines lines Chemistry 101 Unknown Solutions Report Unknown #: Set (circle): A, B, C, D Members of Group 1. 2. Members of Group 3. 4. Contents of Tube: A B C D E Comments (optional): Page 4
Pre-Lab Questions 1. a. Write a balanced equation for the reaction that occurs when an aqueous solution of iron(ii) chloride is mixed with an aqueous solution of potassium hydroxide. b. Write a net ionic equation for the above reaction. 2. What simple test would you use to identify the possible presence of HCl(aq)? What result do you expect if the solution is HCl(aq)? 3. What preliminary test results are you looking for to identify conclusively your unknown solutions as Set C? Explain. 4. When an aqueous solution suspected of containing Ba 2+ or Pb 2+ or both is mixed with sulfuric acid a precipitate forms. In another test, when the original solution is mixed with an aqueous solution of sodium sulfide no precipitate forms. What do these two tests indicate about the likely presence of Ba 2+ and Pb 2+ in the original solution? Explain your answer. Page 5