Solubility and Qualitative Analysis

Lab 2 Name_____________________________ Solubility and Qualitative Analysis Experimental Questions How can solutions containing a mixture of ions be...
Author: Philip Collins
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Lab 2

Name_____________________________

Solubility and Qualitative Analysis Experimental Questions How can solutions containing a mixture of ions be analyzed using solubility tests? Learning goals Review Lewis structures, polarity, and chemical reactions. Learn the chemical reaction to form biodiesel.

Background Some ionic compounds such as sodium chloride will dissolve in water to form a homogeneous mixture called a solution. Solutions are always clear, and very often colorless, so that it is difficult to tell what solutes they contain just by looking at them. One method of solute analysis involves adding a reagent to the solution that causes the solute to precipitate as a solid. Silver nitrate is quite soluble in water at room temperature and forms a clear and colorless solution of aqueous [Ag+1] and [NO3-1] ions. If a solution containing aqueous NaCl is added, a double replacement reaction occurs and the silver ions react with the chloride ions to form solid AgCl. Silver chloride has a very low solubility in water and as a result the formation of a white cloudy precipitate is observed. NaNO3 is soluble in water and remain invisible to the observer. The full ionic reaction equation is: Ag+1 (aq) + NO3-1 (aq) + Na+1 (aq) + Cl-1 (aq) → AgCl (s) + Na+1 (aq) + NO3-1 (aq) The net ionic reaction equation is: Ag+1 (aq) + Cl-1 (aq) → AgCl (s) Other ions will form white cloudy solids when Cl-1 ion is present. Hg+1 ion is one example. If a mixture of mercury [I] and silver ions is present in a solution, adding the HCl will not give a definitive result since two precipitates will form. Another test must be used to distinguish between the two ions. In this lab, various acids, bases, and other reagents will be used to test solutions containing the following cations: NH4+1, Ag +1, Hg+1, Hg+2, and Zn+2 to determine conditions needed to form a precipitate. CAUTIONS: The solutions of the heavy metal ions used in this lab should be considered toxic and care must be taken in handling them. Dispose of wastes in properly marked containers, NOT DOWN THE SINK. Wash hands thoroughly before leaving the lab. The solutions of 6M HCl, 6M NaOH, and 6M NH4OH are strong enough to be considered caustic and should be handled with appropriate regard to safety. WEAR APPROPRIATE EYE PROTECTION AT ALL TIMES. If you spill anything on your person, rinse thoroughly with water. Notify the instructor.

General Chemistry

Lab 2: Solubility and Qualitative Analysis

Procedure Each team member will record direct lab data in INK on this handout. Be sure to make detailed qualitative observations of reagents before, during and after mixing chemicals. Answer the Critical Thinking Questions in order as they are listed during the procedure.

I. Preparation. Obtain a kit containing the following reagents: 6M HCl, 6M NaOH, 6M NH4OH, SnCl2, red and blue litmus paper. Keep a wash bottle of deionized (distilled) water handy. Use a large beaker as a waste receptacle. Assume that 20 drops = 1 mL. Use a pipet (eyedropper) to deliver approximate amounts of test reagents. It is not necessary to measure out in a graduated cylinder. 1. Wash and label 6 large test tubes and put them in a beaker to carry them to the common area. Do not use the test tube rack for transporting test tubes. 2. Place 2-3 mL of each of the following known solutions into five separate labeled test tubes: NH4+, Ag+ , Hg+1, Hg+2, Zn+2. Arrange the samples in a test tube rack after you get back to your station.

II. Analytical Experiments Be sure to keep track of what substances react and what does not react. A. Test for the Presence of NH4+ ions in solution 1. Place approx. 1-2 mL of the NH4+ solution into a clean evaporating dish. 2. Moisten a piece of red litmus paper with the wash bottle and attach it to the underside of a clean watch glass. 3. Use a pipet to add approx. 2 mL of 6M NaOH to the solution in the evaporating dish. 4. Place the watch glass on the evaporating dish with the litmus paper down, and watch for any color change in the litmus. A change from red litmus to blue indicates the presence of NH4+ ion. (If no color change occurs within a few minutes, consult the instructor. You may have to use a hot plate to warm the solution in the evaporating dish.) The reaction shows that ammonium ion in the presence of NaOH will release ammonia gas. NH4+ (aq) + OH- (aq) NH3 (g) + H2O (l) Ammonia gas is basic enough to cause the red litmus to turn blue. 5. Empty all mixtures into your waste beaker. 6. Rinse glassware and evaporating dishes with a wash bottle, making sure all precipitates are drained into the waste beaker. Observations

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General Chemistry

Lab 2: Solubility and Qualitative Analysis

B. Testing the Metal Ions: Ag+, Hg+, Hg2+, Zn2+ 1. Place the test tubes containing the four metal ions in a test tube rack. 2. Add 5 drops of 6M HCl to each of the solutions in the rack. The formation of a cloudy precipitate indicates the presence of an insoluble chloride compound. 3. Note which two metal ions form precipitates, and write a brief description of each precipitate. Observations

C. Testing Ions with a Positive HCl Reaction Set aside samples that did NOT react with HCl. Test the 2 metal ion precipitates that reacted with HCl as follows: 1. Add 5 drops of 6M NH4OH solution to each test tube. Note which ion changes color and describe the product in “Observations” below. 2. To the ion that does NOT change color, add just enough NH4OH to completely dissolve the solid, then add enough HCl to re-precipitate the solid. Note the identity of this ion and describe the precipitate in the solution. Describe any changes that occur in the gas phase ABOVE the solution. (This reaction is occurring between the HCl and NH4OH. The metal ions are not involved.) 3. Empty mixtures from steps 1 and 2 into your waste beaker. Save unreacted samples for part D. 4. Rinse glassware and evaporating dishes with a wash bottle, making sure all precipitates are drained into the waste beaker. Observations

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General Chemistry

Lab 2: Solubility and Qualitative Analysis

D. Test the metal Ions that did NOT react with HCl. 1. Record the identities of the two metal ions that did not precipitate with HCl in Part B. 2. Place the two test tubes containing the non-reacting ions into a test tube rack. 3. Add 6M NaOH to each test tube, 3 drops at a time, until a colored solid appears. (This precipitate may redissolve when the test tube is agitated.) Record which precipitate has color. Record a description of the precipitate. Redissolve this precipitate with 6M HCl. Add several drops of SnCl2. (This is a reducing agent and will lower the charge of the ion.) Add several drops of 6M NH4OH. Record observations. Observations

4. The final ion forms a gelatinous, white precipitate. Sometimes it is difficult to isolate. Avoid adding too much NaOH or HCl at any time. 5. Add more 6M NaOH one drop at a time, shaking gently after each addition until the precipitate appears and remains. Record description of the process. If you already have a precipitate, go to the next step. 6. Add 3-6 more drops of 6M NaOH are added with gentle agitation until the precipitate redissolves. Place one piece of red litmus and one piece of blue litmus onto a watch glass (round side down). Use a glass stirring rod to place one drop of the test tube mixture onto each of the litmus papers. Record results. 7. Add 6M HCl drop by drop, with gentle shaking, until the precipitate reappears. Record observations. 8. Keep adding 6M HCl, drop by drop, with shaking, until the precipitate redissolves. 9. Test with the litmus papers on the watch glass as before and record results. An ion that precipitates if the solution is neutral, but dissolves if the solution is acidic or basic is called amphoteric. This is a rare chemical behavior, and it is repeatable. 10. 6M NaOH can be added at this point to cause the precipitate to reappear. More 6M NaOH will cause the precipitate the redissolve. 6M HCl will then reprecipitate the gel. More 6M HCl will cause it to redissolve. 11. Repeat this cycle twice. Watch the volume in the test tube. If the mixture reaches the halfway point in the test tube, pour some out into the waste beaker before continuing with the test. Record the identity of the amphoteric ion. Describe the process. Observations (continued on next page)

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General Chemistry

Lab 2: Solubility and Qualitative Analysis

Observations

E. Final Clean Up 1. Empty all mixtures into your waste beaker. 2. Rinse glassware and evaporating dishes with a wash bottle, making sure all precipitates are drained into the waste beaker. 3. Rinse the funnels lightly with a wash bottle, allowing the water to drain into the waste beaker. 4. Take the waste beaker and empty the contents into the waste container in the common area. Rinse your waste beaker with a wash bottle to make sure all solids are transferred. 5. Wash all equipment thoroughly before replacing in drawer. 6. Wipe down the benchtop with a wet paper towel. Wash your hands. 7. Return materials to community area. 8. Do NOT remove goggles until ALL GROUPS have finished cleaning up. 9. Return the key and obtain instructor’s initials before leaving.

Instructor Initials ____________

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General Chemistry

Lab 2: Solubility and Qualitative Analysis

Data Analysis and Questions Data Analysis and questions may be completed in pencil. Neatness counts!

Results Table NH4+ test result: Ions

Initially Reacted with

Description of Ppt.

Further Test Reactions

Answer the following questions on a separate sheet and attach to your lab report. 1. Why is it necessary to test for NH4+ ion separately in an evaporating dish? 2. Write the balanced chemical equation that causes litmus to change color in the test for ammonium ion. 3. Write the net ionic equations for each reaction in which a cation reacted with chloride ion. 4. Write the net ionic equations for each reaction in which a cation reacted with hydroxide ion. 5. State the identity of the amphoteric ion and summarize the observations that led to your conclusion. 6. If HCl were added to a mixture containing Ag+, NH4+, and Hg2+, what results would be observed? (State the macroscopic observation you would expect, not the particle-level explanation.) 7. What test procedure would allow you to distinguish between Zn2+ ion and Hg2+ ion? 8. SnCl2 is a reducing agent. Write an appropriate balanced chemical equation for the reaction of SnCl2 and the ion you tested that that reacted with it. 9. Explain the series of tests you would use and the results you would expect to obtain if you tested a solution containing a mixture of NH4+, Hg+, and Zn2+ ions.

Lab Report due next week: Turn in this completed handout stapled in order. Do not re-copy original lab data (up to instructor initials).

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