Identification of a compound by mass relationships lab answers

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CHEM 403 Exp 6 Experiment 6: IDENTIFICATION OF A COMPOUND BY MASS RELATIONSHIPS* In the previous experiment, you learned how to find the formula of a compound by analysis for the elements it contains. When chemical reactions occur, there is a relationship between the masses of the reactants and products that follows directly from the balanced equation for the reaction and the molar masses of the species that are involved. In this experiment, you will use this relationship to identify an unknown substance. Your unknown will be one of the following compounds, all of which are salts: NaHCO3 Na2CO3 KHCO3 K2CO3 In the first part of the experiment, you will be heating a weighed sample of your compound in a crucible. If your sample is a carbonate, there will be no chemical reaction that occurs, but any small amount of adsorbed water will be driven off. If your sample is a hydrogen carbonate, it will decompose by the following reaction, using NaHCO3 as the example: 2NaHCO3(s) → Na2CO3(s) + H2O(g) + CO2(g) (1) In this case, there will be an appreciable decrease in mass, since some of the products will be driven off as gases. If such a mass decrease occurs, you have strong evidence that your sample is a hydrogen carbonate, either NaHCO3 or KHCO3. In the second part of the experiment, you will treat the solid carbonate in the crucible with HCl, hydrochloric acid. There will be considerable effervescence as CO 2 gas is evolved; the reaction that occurs, again using Na2CO3 as our example, is: Na2CO3(s) + 2H+(aq) + 2Cl−(aq) → 2NaCl(s) + H2O(l) + CO2(g) (2) (Since HCl in solution exists as ions, the equation is written in terms of ions.) You will then heat the crucible strongly to drive off any excess HCl and any water that is present, obtaining pure, dry, solid NaCl as your product. To identify your unknown, you will need to find the molar masses of the possible reactants and final products. For each of the possible unknowns there will be a different relationship between the mass of the original sample and the mass of the chloride salt that is produced in Reaction 2. If you know your sample is a carbonate, you need only be concerned with the mass relationships in Reaction 2, and should use as the original mass of your unknown the mass of the carbonate after it has been heated. If you have a hydrogen carbonate, the overall reaction your sample undergoes will be the sum of Reactions 1 and 2. From your experimental data, you will be able to calculate the ratio of the mass of the solid chloride to the mass of either the original hydrogen carbonate or the mass of the anhydrous carbonate in your sample of unknown. From your calculation of the relative masses of solid chloride to solid hydrogen carbonate or solid carbonate in Equations 1 and 2 you can calculate *Adapted from Slowinski, E. J., Wolsey, W. C. Chemical Principles in the Laboratory 9th ed. CHEM 403 Exp 6 what the theoretical ratio of those masses should be. Your observed value should match one of the theoretical values and thus allow you to identify your unknown compound. Procedure Obtain an unknown from the instructor. Weigh a clean, dry crucible, and record its mass. Place approximately 0.5 g of your unknown into the crucible, and record the total mass of the sample with the crucible. Place the crucible on a clay triangle supported by an iron ring. Light your Bunsen burner away from the crucible, and adjust the burner so that you have a small flame. Holding the burner in your hand, gently heat the crucible as you move the burner back and forth. Heat the crucible, gently and intermittently, for a few minutes. Gradually increase the flame intensity, to the point where clay triangle glows red. Heat for 10 minutes. Allow the crucible to cool for 10 minutes, and then weigh it with the sample, recording the mass. At this point, the sample in the crucible is a dry carbonate, since the heating process will convert any hydrogen carbonate to carbonate.