Question

A student attempted to identify an unknown compound by the method used in this experiment. Her original sample weight was 0.7047 g. When heated in the first step, the mass decreased to 0.4862 g Write the complete, net balanced reaction that would occur in this experiment if - the student's unknown compound was NaHCO3 -the student's unknown compound was KHCO3 Using stoichiometry, predict the theoretical yield of the solid chloride obtained at the end of the student's experiment if - the student's unknown compound was NaHCO3 g NaCl 0.7047 g NaHco, x 1 mol NaHCO × mol NaHCO -the student's unknown compound was KHCO3 When the experiment was over, the mass had risen, to 0.5247 g. Did the student do something wrong? Explain why the mass could have gone up. What was the student's unknown compound? Background 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. This is called stoichiometry In this experiment we will use this relationship to identify an unknown substance. Your unknown will be either NaHCO, or KHCO3. Using stoichiometry, you should be able to identify which compound you started with. First, you will be heating a weighed sample of your compound in a crucible. Since your sample is a h carbonate, t will decompose by the following reaction, using NaHCO, as the example ydrogen 2NaHCO3 (s)--Na,CO3(s)-H,O() + CO2 (g) There will be an appreciable decrease in mass during this step, since some of the products will be driven off as gases. Next, we will treat the solid carbonate in the crucible with HC1, hydrochloric acid. There will be considerable effervescence as CO2 gas is evolved; the reaction that occurs is (using Na,CO, as our example) Na CO, (s)+2H(aq)+2C (aq)- 2NaCs)+ H,O0+CO,(9) (Since HCl in solution exists as ions, we write the equation in terms of ions.) The net balanced reaction can be obtained by adding these two reactions together, noting that species that occur on both sides of the reaction can be cancelled out NaHCO(s)+H(aq) Cr (aq) NaCs)+HO+CO.(g) We then heat the crucible strongly to drive off any excess HCl and any water that is present, obtaining pure, dry, solid NaCl as our product. To identify your unknown, you will need to find the molar masses of the possible reactants and final products and do some stoichiometric calculations for each of the two possible unknowns Background 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. This is called stoichiometry In this experiment we will use this relationship to identify an unknown substance. Your unknown will be either NaHCO, or KHCO3. Using stoichiometry, you should be able to identify which compound you started with. First, you will be heating a weighed sample of your compound in a crucible. Since your sample is a h carbonate, t will decompose by the following reaction, using NaHCO, as the example ydrogen 2NaHCO3 (s)--Na,CO3(s)-H,O() + CO2 (g) There will be an appreciable decrease in mass during this step, since some of the products will be driven off as gases. Next, we will treat the solid carbonate in the crucible with HC1, hydrochloric acid. There will be considerable effervescence as CO2 gas is evolved; the reaction that occurs is (using Na,CO, as our example) Na CO, (s)+2H(aq)+2C (aq)- 2NaCs)+ H,O0+CO,(9) (Since HCl in solution exists as ions, we write the equation in terms of ions.) The net balanced reaction can be obtained by adding these two reactions together, noting that species that occur on both sides of the reaction can be cancelled out NaHCO(s)+H(aq) Cr (aq) NaCs)+HO+CO.(g) We then heat the crucible strongly to drive off any excess HCl and any water that is present, obtaining pure, dry, solid NaCl as our product. To identify your unknown, you will need to find the molar masses of the possible reactants and final products and do some stoichiometric calculations for each of the two possible unknowns

Answer 1

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