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ANALYSIS FOR IONS Determination of Cation and Anion of Unknown Salt Although you are most familiar with table salt, which is the salt used in cooking and seasoning of foods, the family of "salts" has many members. Most are found in nature, such as the chlorides of sodium, potassium and calcium, but all can be formed in the laboratory as a product of a neutralization reaction, that is a reaction between an acid and a base in water. The formation of table salt, NaCl, results by reacting hydrochloric acid and sodium hydroxide as represented by the following equation: HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l) Your unknown salt contains only one cation. It may be any one of the following four: Na+, K+, Ca2+ or Fe3+ Your unknown salt contains only one anion. It may be any one of the following four: CO32-, Cl-, SO42- or PO43- Thus, there are many possible combinations, and you must determine which salt is the unknown given to you. 1. Testing for the Cation of the Unknown Salt You should observe only one positive test for the cation in your unknown. a) In a flame tests for Na+, K+, Ca2+, the color of the flame is bright yellow, violet, or orangered, respectively. However, the orange-red flame for calcium may not be very prominent. b) Thus, calcium will require a confirmatory test, and will form a white precipitate when treated with ammonium oxalate (NH4)2C2O4. c) When a salt containing Fe3+ is treated with potassium thiocyanate (KSCN) solution, a bloodred colored solution will form. d) By now you should have determined which cation is present in your unknown. 2. Testing for the Anion of the Unknown Salt You should observe only one positive test for the anion in your unknown. a) Any salt containing CO32- ions will react promptly with an acid (e.g., HCl(aq)) to form carbon dioxide gas, which will foam and bubble up in the test tube. b) If the unknown solution immediately forms a white precipitate when mixed with silver nitrate (AgNO3), and if the precipitate is permanent (that is, it cannot be dissolved in nitric acid), then it proves that chloride ion (Cl-) is present. c) Sulfate ions will form a permanent white precipitate when treated with barium chloride (BaCl2(aq)) and nitric acid (HNO3). d) The proof that PO43- ions are present is the formation of a bright yellow precipitate when a phosphate salt is treated with ammonium molybdate, ((NH4)2MoO4) and nitric acid (HNO3). precipitate. A positive result would indicate which anion? 2 e) By now you should have determined what anion is present in your unknown. 3. Conclusion The report form contains observations that were made by one of your classmates. Based on these observations, fill in the blanks to indicate if the ions are present or absent. From the overall set of observations decide which of the salts is your unknown by giving its cation and anion, formula, and name at the end of the report sheet. 3 REPORT FORM Experiment #9 – Analysis for Ions Unknown Sample # ______XYZ__________ PART A: CATION TESTS OPERATION OBSERVATION CONCLUSION Section 1: Flame test for Na+, K+ and Ca2+ ions NaCl Bright yellow KCl Lavender CaCl2 Not very evident Flame test for unknown Bright yellow Section 2: Test for Ca2+ ions (NH4)2C2O4 with NaCl Clear solution (NH4)2C2O4 with KCl Clear solution (NH4)2C2O4 with CaCl2 White precipitate (NH4)2C2O4 with FeCl3 Clear solution (NH4)2C2O4 with unknown Clear solution Section 2: Test for Fe3+ ions KSCN with NaCl Clear solution KSCN with KCl Clear solution KSCN with CaCl2 Clear solution KSCN with FeCl3 Blood red precipitate KSCN with unknown Clear solution The unknown cation is ___________________ PART B: ANION TESTS 4 OPERATION OBSERVATION CONCLUSION Section 4: Test for Carbonate (CO32-) ions 6 M HCl with Na2CO3 Bubbling 6 M HCl with NaCl Clear solution 6 M HCl with Na2SO4 Clear solution 6 M HCl with Na3PO4 Clear solution 6 M HCl with unknown Clear solution Section 5: Test for Chloride (Cl-) ions After HNO3 added AgNO3 with Na2CO3 / HNO3 White precipitate Clear solution AgNO3 with NaCl / HNO3 White precipitate Precipitate stays AgNO3 with Na2SO4 / HNO3 Clear solution Clear solution AgNO3 with Na3PO4 / HNO3 White precipitate Clear solution AgNO3 with unknown / HNO3 White precipitate Clear solution Section 6: Test for Sulfate (SO42-) ions After HNO3 added BaCl2 with Na2CO3 / HNO3 White precipitate Clear solution BaCl2 with NaCl / HNO3 Clear solution Clear solution BaCl2 with Na2SO4 / HNO3 White precipitate Precipitate stays BaCl2 with Na3PO4 / HNO3 White precipitate Clear solution BaCl2 with unknown / HNO3 White precipitate Precipitate stays Section 7: Test for Phosphate (PO43-) ions (NH4)2MoO4 with Na2CO3 Clear solution (NH4)2MoO4 with NaCl Clear solution (NH4)2MoO4 with Na2SO4 Clear solution (NH4)2MoO4 with Na3PO4 Yellow precipitate (NH4)2MoO4 with unknown Clear solution The unknown anion is _______________ Summary from Observations and Conclusions in PARTS A and B: Unknown Sample # ____XYZ______ Cation __________ Anion ___________ Unknown Name __________________________ Unknown Formula______________ 5 Supplemental Questions Answer the following questions and submit them along with your Report Form. 1. Write an equation for the main chemical reaction used to test for each: 9a) CO32- ________________________________________________ b) Cl- ________________________________________________ c) SO42- ________________________________________________ 2. Write an equation for the main chemical reaction to test for: a) Ca2+ _________________________________________________ b) Fe3+ _________________________________________________ 3. In the space below describe how you tested for Na+ and K+ ions. 4. A student was given an unknown solid that gave the following test results: A portion of the solution of unknown emitted bubbles of a colorless, odorless gas when hydrochloric acid solution was added. Another portion of the solution gave a blood-red colored solution with potassium thiocyanate solution. a) Which ions are present? Cation ______ Anion ______ b) Name the unknown salt. __________________________ c) Write the correct formula for the salt. _________________ 5. Another student's unknown gave the following test results: a portion of the solution of unknown gave a yellow precipitate with ammonium molybdate reagent. A flame test on some of the solution resulted in a bright yellow flame. a) Which ions are present? Cation ______ Anion ______ b) Name the unknown salt. ___________________________ c) Write the correct formula for the salt. _________________ 1 MELTING POINT DETERMINATION OF KNOWN AND UNKNOWN SUBSTANCES INTRODUCTION Melting point (MP) can be defined as the temperature at which there is a change of state from solid to liquid, and when these two phases are in dynamic equilibrium. In the case of phase change, dynamic equilibrium means that the processes of melting and solidification are occurring at the same time. Actually, melting point is not a single temperature value, but rather a melting range, that is, the temperature interval or range at which a solid completely changes from solid to liquid. A pure substance has an expected narrow melting range of 1.0 to 3.0 °C. For example, the reported MP of pure benzoic acid is 121-122°C. The presence of impurities would probably lower the melting point and widen its range to between 105-120°C, depending on the quantity of impurity present. The melting behavior of a sample thus gives a clue about its purity. Watch the brief video included here to illustrate how melting point is determined and what you would observe as a solid melts. https://www.youtube.com/watch?v=8T0OSYI5pYk The determination of the melting point is used, among other tests, to identify a compound and to obtain evidence of its purity. Standard reference literature contains long lists of carefully measured physical properties, which include melting and boiling points for both organic and inorganic compounds. In this experimental simulation, you will be given 3 unknown compounds. Comparisons of the MPs of your samples with those listed in Table 1, would narrow the list of substances that your compound might be. Although only a few are on the list, there can be many substances that may have very similar melting points to those of your unknowns. The identity of an unknown compound may be determined by a method called the mixed melting point technique. It is used in this experiment to determine if two samples are identical or not. The requirement is that you are able to obtain a pure sample of the substance that you suspect your unknown sample to be. If a known substance is mixed together with your unknown, and there is no change in the mixed MP, you can assume that your unknown and the known substance are the same. But, if the two samples are, in fact, different substances, then there would be a marked difference in their mixed MP. This is because one substance behaves like an impurity in the other. Impurities will lower the melting point temperature and widen the melting range when compared to the melting point of the pure substance. Note: Only impurities that are soluble in the solid can impact the MP. Impurities such as paper fibers and dust have no effect on the MP. In this process, an unknown is mixed together in equal quantities with a known compound, and the mixture’s MP determined. If the mixture melts at a lower temperature than the known compound or if the melting range is broad, then the known and unknown are not the same. If the mixture melts at the same temperature and range as the known compound, then the unknown and known are the same. Read the attached hand written lab notes provided by your former lab partner, who left college to join an expedition in search of lost cities, where there are no lines of communications. Your challenge is to identify the 3 unknown compounds by using the notes she left behind. PROCEDURE Melting Point Experiment 2 I. Melting Points of Pure Substances 1. In this simulated experiment, you are given 3 samples (A, B, and C) of pure organic compounds, whose melting point ranges are: A. 79-81°C B. 111-113°C C. 165 to 167°C 2. Record these values in the proper blank spaces on top of page 4 of the attached report form. 3. In the Section, "Experimental Data" on page 4, record the names and melting point values of all the candidate compounds from Table 1 that fall into the same melting ranges of each of the unknowns. II. Mixed Melting Points 1. Carefully read your partner's notes that are in the Appendix. They contain the observations about her experiments studying the melting point ranges of samples A, B, and C when they were mixed with the candidate organic compounds listed in Table 1. 2. To help organize your thoughts about this experiment, collect the mixed melting point data from these lab notes and enter them into the appropriate spaces in the "Experimental Data" section of the attached report form. You must complete one form for each unknown on the Summary Sheets on pages 4 and 5. III. The Identification of An Unknown Substance by Mixed Melting Point Technique 1. For each unknown, you will learn that one of the mixed melting point ranges is narrow and close to the literature value of the unknown. The other mixed melting point ranges are broad and lower than the literature values. 2. Now you can identify your unknown samples by writing their names after each Summary Sheet in the blank lines, "Identity of Unknowns A, B, and C". IV. Deliverable Answer the supplemental questions on page 5, and submit it along with the report forms on pages 3 to 5 for this experiment. Melting Point Experiment 3 Melting Point Determination Report Form Name_________________________________ Section ______________ Date ____________ Table of Known Compounds Table 1 lists 33 pure compounds, several of which you will select to mix with each of your unknowns, so that you can determine the melting points of the mixtures, and thus the identity of the unknowns. Table 1. Known Compounds for Melting Point Experiment COMPOUND mp range (oC) COMPOUND mp range (oC) 1) 1,4-dimethoxybenzene 54-56 21) 4-nitroaniline 146-149 2) 2-phenylphenol 57-59 22) 1-napthoic acid 158-160 3) diphenylacetylene 59-61 23) salicylic acid 159-161 4) 2,4,6-trichlorophenol 64-66 24) triphenylmethanol 160-162 5) biphenyl 68-70 25) sulfanilamide 165-167 6) methyl 3-nitrobenzoate 78-80 26) 4-bromoacetanilide 165-169 7) naphthalene 80-82 27) itaconic acid 165-168 8) vanillin 81-83 28) d-mannitol 166-168 9) acetamide 79-81 29) acetaminophen 168-172 10) phenoxyacetic acid 98-100 30) succinic acid 184-186 11) resorcinol 109-112 31) hippuric acid 187-191 12) 3-toluic acid 111-113 32) 4-terphenyl 212-213 13) acetanilide 113-115 33) anthracene 210-215 14) 4-hydroxybenzaldehyde 112-116 15) (+/-)-mandelic acid 119-121 16) benzoic acid 122 17) 2-naphthol 120-122 18) 2-benzoylbenzoic acid 126-129 19) trans-cinnamic acid 132-135 20) urea 132-135 References: 1) Aldrich Handbook of Fine Chemicals, 2007-2008; 2) The Merck Index, 12th edition; Budavari, S., Ed.; Merck & Co., Inc.: Whitehouse Sta., NJ, 1996. Melting Point Experiment 4 In the blank spaces below record the names and the literature melting points of the 3 unknown compounds available to you. List these unknowns in order of increasing MP. Labels of the Unknowns Literature MP A °C B °C C °C Experimental Data Mixed Melting Points Summary Sheets For each unknown, fill in the data taken from your lab partner's notes in the Appendix. Unknown A: Melting point of Unknown A: _________________ Candidate compounds from Table 1: 1. ________________________________________________ M.P. ____________ 2. ________________________________________________ M.P. ____________ 3. ________________________________________________ M.P. ____________ 4. ________________________________________________ M.P. ____________ Mixed melting point ranges with unknown A: Unknown + 1: ____________________________________________ M.P. ____________ Unknown + 2: ____________________________________________ M.P. ____________ Unknown + 3: ____________________________________________ M.P. ____________ Unknown + 4: ____________________________________________ M.P. ____________ Identity of Unknown A: _____________________________________ Unknown B: Melting point of Unknown B: _________________ Candidate compounds from Table 1: 1. ________________________________________________ M.P. ____________ 2. ________________________________________________ M.P. ____________ 3. ________________________________________________ M.P. ____________ 4. ________________________________________________ M.P. ____________ Mixed melting point ranges with Unknown B: Unknown + 1: ____________________________________________ M.P. ____________ Unknown + 2: ____________________________________________ M.P. ____________ Unknown + 3: ____________________________________________ M.P. ____________ Unknown + 4: ____________________________________________ M.P. ____________ Identity of Unknown B: _____________________________________ Melting Point Experiment 5 Unknown C: Melting point of Unknown C: _________________ Candidate compounds from Table 1: 1. ________________________________________________ M.P. ____________ 2. ________________________________________________ M.P. ____________ 3. ________________________________________________ M.P. ____________ 4. ________________________________________________ M.P. ____________ Mixed melting point ranges with Unknown C: Unknown + 1: ____________________________________________ M.P. ____________ Unknown + 2: ____________________________________________ M.P. ____________ Unknown + 3: ____________________________________________ M.P. ____________ Unknown + 4: ____________________________________________ M.P. ____________ Identity of Unknown C: _____________________________________ Complete the following exercises in the spaces provided: 1. Describe the effects of impurities on the melting point of a pure substance. 2. Based on your experience with this simulated experiment, what is a reasonable definition of a pure substance. 3. An unknown solid has a melting point of 132-135°C. A mixed melting point of the unknown with urea, gives a melting point of 120-126°C. Based on your understanding of the effects of impurities on melting points, is the unknown urea or not? Briefly explain your answer. 4. a) An unknown solid has a melting point of 112-114°C. Based on data from Table 1 above and the mixed melting point technique of this experiment, you can narrow down the possibilities to three compounds. List the compounds. b) A mixed melting point of the unknown with acetanilide gives a melting point of 113-114°C. Is the unknown acetanilide or not? Briefly explain your answer.

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