Separation and Qualitative Determination of Cations and AnionsInstructor’s Guide Master Materials List (complete for 24 students working in pairs) Silver nitrate, AgNO3, 0.2 M, 100 mL* Silver nitrate, AgNO3, 0.1 M, 100 mL* Cupric nitrate, Cu(No3)2, .02 M, 100 mL* Zinc nitrate, Zn(NO3)2, 0.2 M, 100 mL* Ferric nitrate, Zn(NO3)2, 0.2 M, 100 mL* Hydrochloric acid, HCl, 6 M, 125 mL* Nitric acid, HNO3, 6 M, 125 mL* Ammonia, NH3, 6 M, 125 mL* Sulfuric acid, H2SO4, 6 M, 125 mL* Acetic acid, CH3COOH, 6 M, 125 mL* Sodium chloride, NaCl, 0.2 M, 100 mL* *Materials included in kit Sodium carbonate, Na2CO3, 0.2 M, 100 mL* Sodium hydroxide, 6 M, 100 mL* Sodium sulfate, Na2SO4, 0.2 M, 100 mL* Sodium nitrate, NaNO3, 0.2 M, 100 mL* Barium chloride, BaCl2, 0.1 M, 100 mL* Barium hydroxide, Ba(OH)2, 5 g* Potassium thiocynate, KSCN, 0.1 M, 100 mL* Potassium ferrocyanide, K4Fe(CN)6, 0.1 M, 140 mL* Aluminum granules, Al, 50 g* Beral-type pipets, 120* Additional Materials Needed Beakers, 400-mL, 12 Beakers, 250-mL, 5-14 Centrifuges, 3 Dropper bottles, 15-30 mL, 96 Hot plates, 3-12 Litmus paper or pH paper Microscope slides or watch glasses, 12 Spatulas, 12 Stirring rods, 12 Stoppers to fit test tubes, 72 Storage bottle, with cap, 125 mL Test tubes, 13 X 100 mm, 72 Wire test tube holders, 12 Preparation of Solutions (for 12 student groups) Note: 0.2 M solutions are provided so unknown solutions can be easily made. The 0.2 M solutions are also easier to ship and take up less room. Dilution the 0.2 M solutions to 0.05 M solutions does not have to quantitative. Use the markings on the beaker for measurements. Solution of Four Cations Prepare 100 mL of a 0.05 M solution of all the cations to be tested by adding 25 mL of each of the following solutions to a clean 250-mL beaker. 0.2 M silver nitrate, AgNO3 0.2 M cupric nitrate, Cu(NO)3 0.2 M zinc nitrate, Zn(NO3)2 0.2 M ferric nitrate, Fe(NO3)3 Mix the solution, dispense in 12 clean 13 X 100 mm test tubes, and a stopper the test tubes. Label as known cation solution. Unknown Cation Solutions Prepare unknown cation solutions for the students to analyze from the four 0.2 M stock solutions of cations. Any combination of these four cations may be prepared. Using a graduated Beral-type pipet, add 2 mL of each ion selected for the unknown to a clean 13 X 100 mm test tube, then add distilled water to create a total volume of 8 mL. Stopper the test tube. Some suggested combinations appear on the table below. “Ba(OH) 2” (saturated. However. and stopper the test tubes.2 M sodium chloride. Add distilled water to give a total volume of 8 mL. 1 2 3 4 5 6 7 8 9 10 11 ClX X X X X X X CO32X X X X X X X X X X X X X NO3X X X SO42X X X X X Barium Hydroxide Solution Prepare 100 mL of a saturated barium hydroxide solution by dissolving 5. 0. Silver nitrate solution is mildly toxic and irritating to body tissue.2 M sodium sulfate. Transfer the 100 mL of solution to a stoppered or capped bottle and label. A matrix of combinations. NaCl 0.05 M solutions are very dilute and hazards are greatly reduced. Unknown No. can be generated for the anions. Na2SO4 Mix the solution. Cupric nitrate solution . Na2CO3 0. similar to the cation unknowns. dilute silver salts will still stain the skin. Stopper the test tube. add 2 mL of each ion selected to a 13 X 100 mm test tube. Unknown Anion Solutions Prepare the unknown anion solutions for the students to analyze from the four 0. Using a graduated Beral-type pipet.2 M sodium carbonate. dispense in 12 clean 13 X 100 mm test tubes. NaNO3 0. Any combination of these four anions may be prepared.05 M solution of all the anions to be tested by adding 25 mL of each of the following solutions to a clean 250-mL beaker.0 g of barium hydroxide in 100 mL of deionized or distilled water. It also stains skin and clothing.2 M sodium nitrate.Unknown No. 1 2 3 4 5 6 7 8 9 10 11 Ag+ X X X X X X X Cu2+ X X X X X X X Zn2+ X X X Fe3+ X X X X X X X X X X X Solution of Four Anions Prepare 100 mL of a 0.2 M stock solutions of anions.) Safety Precautions The 0. separating the liquids from solids.is mildly toxic. Check centrifuges to make sure that they are balanced and spin quietly while empty. skin. This waste may be disposed of according to Flinn Suggested Disposal Method #27h. and are corrosive to all body tissues. concentrated acids and bases from the stock solutions. To simplify preparation of the sets of dropper bottles. Concentrated ammonia (ammonium hydroxide) solution and hydrochloric acid solutions are toxic by inhalation. precipitating the substances. chemical-resistant gloves. Each liquid may be disposed of according to Flinn Suggested Disposal Method #26b. Acetic acid solution is a corrosive liquid. The test tubes should not drop so far into the centrifuge tubes that they cannot be easily removed. One hour of prep time will be needed to prepare the known and unknown solutions. Dispoal Provide a separate container for cations and for anions into which the students may empty their waste solutions. Students should have a set of reagents available to them in small (15-39 mL) dropping bottles. . a strong oxidant. Dispense the reagents from the stock solutions. If a complete set of dropper bottles is not available for each student group. Ferric nitrate solution is irritating to body tissue. Please consult current Material Safety Data Sheets for additional safety information. and toxic by ingestion and inhalation. It is essential to maintain a good record of the results of each step because the procedure will be carried out over a number of laboratory periods. Great care is needed in adding reagents. If necessary. Wear chemical splash goggles and chemical-resistant gloves. AP Chemistry Standards This lab fulfills the requirements for the College Board recommended AP Experiment #14: Separation and Qualitative Analysis of Cations and Anions. mixing solutions. place a small cork in the bottom of each centrifuge tube so that the test tubes extend just a little above the centrifuge tubes. For barium solutions and solids from anion steps 2 and 3. Potassium thyocyanate solution is slightly toxic by ingestion. Wear chemical splash goggles. Sulfuric acid solution and sodium hydroxide solution are severely corrosive to eyes. • • • Teaching Tip • The solution that is analyzed has a small total solution volume. place in a separate waste container. Filter the solids from the liquid. washing precipitates. Enough solution is provided for each lab group to perform multiple tests. ingestion. Zinc nitrate solution is mildly toxic and is irritating to body tissue. have each student group clean and label a set of dropper bottles for the reagents. Have students wash hands thoroughly with soap and water before leaving the laboratory. Please consult your current Flinn Scientific Catalog/Reference Manual for proper disposal procedures. Avoid contact with concentrated acids since toxic hydrogen cyanide gas may be liberated. Water baths can be shared. and other tissue. adjusting pH. Nitric acid solution is severely corrosive. Barium hydroxide and barium chloride solutions are toxic by ingestion. Use great caution in dispensing the strong. Lab Hints • • Four to five 50-minute lab periods are needed to analyze the known and unknown solutions for both cations and anions. The cation solids may be disposed of according to Flinn Suggestion Disposal Method #26a. a set may be shared between groups. and following the laboratory procedures. It may be necessary to weigh the centrifuge tubes and place those with the closest weights opposite on another. It is not necessary to stopper the test tube when centrifuging the solution. Caution students not to stop the centrifuge while it is spinning. Students should be able to demonstrate an understanding of the chemistry involved in the steps as well as the procedures used. and a chemical-resistant apron. Encourage students to run tests until they get clear and reproducible results. A test tube must be counterbalanced by placing a similar tube with an equal volume of water opposite it. and ClSome 6 M HCl is added to the solution that may contain the above ions. Ions present: _________ Ions absent: __Ag+. Fe3+. Test for CO32. NO3-__ Only one of each of the following pairs of reactants undergoes a reaction. The dark precipitate completely dissolves in 6 M H2SO4. NaCl(aq) + BaCl2(aq) → no reaction Na2SO4(aq) + BaCl2(aq) → BaSO4(s) + 2Na+(aq) + 2Cl-(aq) 7. Fe3+ Some 6 M HCl is added to a solution that may contain the three ions. The addition of 6 M NH3 to the acidic solution until it is basic results in a clear solution containing a dark precipitate. and Zn3+ 6 M NaOH is added to a clear solution that may contain the three ions until the solution is basic. and Zn2+ Some 6 M HCl is added to a solution that may contain the three ions. Formation of bubbles is noted as the solution is heated. No precipitate forms. Ag+. The addition of 6 M NaOH until the solution is basic results in no formation of the precipitate. Test for Cu2+. Test for Cl-. SO42-. Cu2+. A white precipitate forms. Cu2+__ Ions undetermined: ____Zn2+___ 3. 6.Answers to Pre-Lab Questions 1. K+(aq) + Cl-(aq) → no reaction Ag+(aq) + Cl-(aq) → AgCl(s) . The precipitate totally dissolves in 6 M H 2SO4. Test for Ag+. Addition of BaCl2 to fresh solution also causes no precipitate to form. SO42-___ Ions undetermined: _CO32-. Ions: ____Ag+___ Ions absent: __________ Ions undetermined: __Cu2+. Complete and balance the equation for the reaction which occurs. and NO3Addition of AgNO3 causes no precipitate to form. Fe3+__ 2. CO32-. Ions present: ___Fe3+____ Ions absent: ____Cu2+____ Ions undetermined: ____Cl-____ 4. A dark precipitate forms. Test for Cu2+. NO3-(aq) + OH-(aq) + Al(s) → no reaction 3NO3-(aq) + 5OH-(aq) + 8Al(s) + 18 H2O(l) → 3NH3(aq) + 8Al(OH)4-(aq) CO32-(aq) + OH-(aq) + Al(s) → no reaction 8. Ions present: ___CO32-____ Ions absent: __________ Ions undetermined: ___Zn2+___ 5. Ions present: _______ Ions absent: ___Cl-. Centrifuge. Pour colored solution into TT2. Precipitate forms Ag+ present Unknown Solution Results Conclusion 3 Cu2+ and Fe3+ may be present 4 Basic solution blue Cu2+ present 5 Acidic solution turns deep red Fe3+ present 6 White to light green Zn2+ present ppt Anion Analysis Data Table (Student Answers will vary. Pour solution in TT3. Add 0. Add K4[Fe(CN)6]. Solution turns deep red. Heat. Capture gas in pipet. Discard water. Add 0. Stir to dissolve ppt. Precipitate forms. Add 10 drops of 0. Precipitate forms. Litmus gradually turns blue.5 mL of 6 M HNO3. Known Solution Results Conclusion White ppt forms Cl. discard blue solution. Acidify solution with 6 M HNO3. add NaOH. Add 0. Centrifuge. Solution blue.Sample Data Tables Cation Analysis Data Table (Student answers will vary) Step 1 2 Procedure Add HCl to known solution in TT1.0 mL of known solution to TT5.5 mL BaCl2. White ppt forms. Add NH3. white ppt with HCl. Precipitate forms. Solution basic. Add H2SO4 until ppt dissolves. Add HCl to solution in TT3 (step 3). Add 1. Wash ppt in TT2.present Unknown Solution Results Conclusion 2 White ppt forms White ppt forms CO32. Solution acidic.5 mL of NH3. Centrifuge. Cloudy white ppt forms Add 0. Solution acidic.1 M AgNO3. White ppt forms. Heat TT2. Stir. To TT2. Transfer solution to dry TT6. Add 5 drops KSCN.) Step 1 Procedure Add CH3COOH to known solution in TT1. Add HCl.5 mL of known solution to TT4. White to light green ppt forms. add H2SO4 until solution is acidic. Add 2 mL of Ba(OH)2 solution. Centrifuge.present 3 SO42. To solution in TT2. Centrifuge. Heat.present 4 Litmus paper turns from pink NO3. Add 6 M HNO3. Add 0. Centrifuge.5 mL of 6 M HNO3 to known solution in TT2. Dissolve precipitate. Known Solution Results Conclusion White ppt forms Precipitate dissolves in NH3. discard liquid. Centrifuge. Add NaOH and water to ppt in TT2. Add NH3 to precipitate in TT1.present to blue . the precipitation of BaCO3. the precipitate is dissolved. A solution of NaCl would also work as well. H2O. is the confirmatory test for carbonate ion. Write equations for these two reactions. The confirmatory test for chloride ion with silver ion is the same chemical reaction used to confirm silver in the cation analysis scheme. why must care be taken to keep the moist litmus from coming in contact with the cotton or the solution? The solution being tested is basic. One is a precipitate and one a complex ion in solution. Precipitation: Ag+(aq) + Cl-(aq) → AgCl(s) Dissolution: AgCl(aq) + 2NH3(aq) → Ag(NH3)2+(aq) + Cl-(aq) Reprecipitation: Ag(NH3)2+(aq) + Cl-(aq) + 2H+(aq) → AgCl(s) + 2NH4+(aq) 5. Write separate oxidation and reduction half-reactions for the procedure used in the test for nitrate ions. but in acidic solution BaCo3 will decompose to form CO2 gas. In step 4. Why doesn’t BaCO 3 precipitate in step 4 but does in step 3? BaSO4 will precipitate in acidic solution. However. and then AgCl is precipitated again in the confirmatory step. .→ NH3(aq) + Al(OH)4-(aq) Al(s) + 4OH-(aq) → Al(OH)4-(aw + 8ereduction oxidation NO3-(aq) + Al(s) + 5OH-(aq) + 6H2O(l) → NH3(aq) + Al(OH)4-(aq) overall 6. Ba2+ is added to the solution containing all four of the anions and precipitates BaSO 4. NO3-(aq) + 6H2O(l) + 8e.Post-Lab Questions (Student answers will vary. but not BaCO3. In the nitrate test. Explain what the reaction is and how the initial precipitate is dissolved and reprecipitated. Use equations in your explanation. Ag+ is precipitated as AgCl. If the litmus comes in contact with the solution or cotton moist from the solution. Fe3+(aq) + 3NH3(aq) + 3H2O(l) → Fe(OH)3(s) + 3NH4+(aq) Cu2+(aq) + 4NH3(aq) → Cu(NH3)42+(aq) 4. it will turn blue from the solution contact. 2. When Fe3+ and Cu2+ react with NH3 solution they form two different types of products. Explain the chemistry of each of these steps by showing a balanced equation for each. not from the evolving ammonia. in step 3. In the analysis scheme. 7. Precipitation: Dissolution: Reprecipitation: Ag+(aq) + Cl-(aq) →AgCl(s) AgCl(aq) + 2NH3(aq) → Ag(NH3)2(aq) + Cl-(aq) Ag(NH3)2+(aq) + Cl-(aq) + 2H+(aq) → AgCl(s) + 2NH4+(aq) 3.) 1. and Ba2+(aq). What is the precipitating reagent for silver (Ag +)? Would a solution of NaCl work as well? Why or why not? The chloride ion is the precipitating reagent for the silver ion. since it provides the chloride ion in solution.
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