edited by
JOHN J. ALEXANDER Cincinnati. OH 45221
What's Present? Qualitative Analysis Problems in General Chemistry Sidney Toby Rutgers, the State University of New Jersey New Brunswick, NJ 08903 Articles on classical qualitative analysis still appear quite frequently ( 1 4 ) ) ,even though, a s Laing has pointed out (.4.~ ) .the basic method dates back 150 vears. Althoueh qualitative analysis in organic chemistry still flourishes, there is little doubt of the decrease i n inorganic aualitative analysis being done i n current first-year laboratories. There are good reasons for this: conventional qualitative analysis is slow, cumbersome, and outmoded. Nevertheless. attemots occasionallv are made to combine i t with current tcchn~ques,such a i paper chromatography 15,. Anart from th? n(wl to t m n student; in I n h ~ ~ r a ttechll~ niq;es, qualitative analysis has the virtue of teaching students something of the properties of common compounds. The lack of knowledge about common substances often has been pointed out (a famous example of error: "silver chloride is a green gas" (6)).An important part of standard qualitative analysis is that i t involves knowing that, for example, nitrates generally are soluble (the solids occur naturally only in deserts); whereas, most carbonates, other than those of the alkali metals, generally are insoluble (and often occur naturally). Students normally are not enthusiastic about learning the "Solubility Rules" that are given in almost any standard first-year general chemistry textbook because t h e rules are boring to memorize. However, it i s possible to test students on this kind of knowledge as well a s some elementary descriptive chemistry, an; allow them to look up the factual information they need. They still find intriming the detective work needkd to obtainthe answers to the kind of problems listed below. Factual information is needed; but so is logic. The problems increase in difficulty and in all cases the question is: "Which salts MUST be present (P), which MUST be absent (A) and that are indeterminate from the given data (O)? All chemical solutions are aqueous and in moderate concentration. Problems 1. A sample consists of one or more of the following salts: NazC03, KNO3, CaCIz. The sample dissolves completely in water and when dilute HCI is added to this solution effervescence is seen. 2. Asample consists of one or more ofthe following salts: NaCH3COz (acetate), KCI, Hgz(N0dz. The sample dissolves completely in water, but when aqueous potassium iodide is added a precipitate forms. 3. A sample consists of one or more of the following salts: NH4N03, NazS, KzC03, Pb(CH3COz)z. The sample dissolves completely in water, but when aqueous sodium sulfate is added a precipitate forms. 4. Asample consists of one or more of the following salts: NH4N0.3, (NH41zS04,NaN03, KzS, Ba(N0dz. The sample dissolves completely in water, and the solution is divided into three parts. Adding AgN03 solution to one part gives 1008
Journal of Chemical Education
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a black orecioitate insoluble in NHn solution. Heatine a second part with NaOH produces a gas which turned m z s t red litmus DaDer blue and smelled of NH3. The third nart gives a white precipitate with NazS04 solution. 5. A sample consists of one or more of the following salts: NH4N03, NH4Br, Ca(N03)z, AgN03, Pb(N03)z. The sample dissolves completely in water, and when cold dilute HCI solution is added there is a white precipitate that is filtered off and found to be completely soluble in hot water. To the filtrate, (NH4hC03 solution is added and a white precipitate forms. Heating the original solid with NaOH solution gives a gas that turns moist red litmus blue and has a pungent smell. 6. Asample consists of one or more of the following salts: NaCl, KI, BaClz, CuClz, Hgz(N0dz. The sample dissolves completely i n water to give a colorless solution. To a small part, chlorine water and some carbon tetrachloride is added and the mixture is shaken. Aviolet color appears. 7. Asample consists of one or more of the following salts: KzS04, CaC03, Ba(NO&, Cu(N03)z, NaN03. The solid when treated with water gives a blue solution and a white precipitate that is filtered off. The precipitate is a white solid that is completely insoluble both in water and in 6M HCI solution. 8. Asample consists of one or more of the following salts: KCH~COZ, BaCIz, FeC13, AgNO3, Pb(N0dz. The sample is a white solid that does not dissolve completely in water nor i n 6M HCI solution. I t does, however, dissolve completely i n NH3 solution. Responses and Reasons 1. Answer: Na2C03(PI, KN03 (01,CaCl, (A). Reasmine: The effervescence could have been anlv CO., insoluble 2. Answer: KC1 (A),Hg2(NO3I2(PI, NaCH,CO, (01. Reasoning: Mercury(1)is the only cation in this question
to give an insoluble chloride or iodide, so it must be present. But if HgZ2'is present, KC1 must have been absent because the original was completely soluble. :I. . l n . ~ u . r : Z l H : N O ,0 .Sa;S A ,K.CO, .\ .PbC'H,CO: P R r a w n t n ~ Only : 1'1>." thc jomplr forms nn insoluhr sulfue. R u t tfPh'' > sprtserlt, U.e,S:aud K,CO rnus h.wc
been absent because h e sample was com$eteiy soluble. Ba(NO,), (P).
Reasoning: The black precipitate could have been only
A& (silver tarnish is black). Ba(NO& must have been present to give a precipitate wi
