7# &oN$e@L|L|L| L LL<MMMM4MDMZMZzMxMNL NlN*NL<NNNNNNNNNNNExperiment VII Analysis of an Unknown Mixture Using the Ideal Gas Law pre-lab assignment Reading: Before coming to your discussion section, read the following: 1. Sections 5-2 and 5-6 in Olmstead and Williams. 2. The remainder of this experiment in this manual. Pre-lab assignment to hand in: Prepare written responses to the following questions and submit your work to your instructor at the beginning of your discussion period: 1. Why is it important before the initial volume reading to adjust the leveling bulb so that water levels in the leveling bulb and measuring tube are exactly the same? 2. a) What gases are present in the measuring tube, and in the Erlenmeyer flask and tubing as well, at the point of the final volume reading? b) How do you account for these other gases, since you only want to calculate the volume and moles of dry N2 gas collected? 3. For safety reasons described in the procedure, sulfamic acid and sodium nitrite should not be brought together as solids. Nevertheless, what other reason is there for adding water to the vial containing the unknown sample and to the Erlenmeyer flask containing sulfamic acid? 4. Show by appropriate calculations that there is excess HSO3NH2 used in this experiment even if the unknown sample is pure NaNO2. Introduction A mixture is composed of two or more substances that do not react with each other. The possible compositions of some mixtures may vary over a wide range. Mixtures are encountered frequently both in nature and in the laboratory. Salt beds, salt brines, and sea water--major sources of many important salts--are mixtures. In most chemical syntheses, the desired product is part of a mixture composed of other reaction products as well as undesired side products and leftover reactants. Heterogeneous mixtures such as cement, topsoil or the unknown in this experiment can have variable compositions, structures, and physical and chemical properties such that abrupt discontinuities or boundaries may be observed within the mixture. One of the methods of determining the percent composition by mass of a mixture of two components is to measure quantitatively the amount of product formed by the reaction of one of the components under conditions where the other component is inert and remains unreacted. If one of the products is a gas, that gas can be collected, its volume can be determined under measured conditions of pressure and temperature, and the number of moles of gaseous product can be calculated using the Ideal Gas Equation. The moles of product can be related to the moles and mass of the reactant from which the product was formed. By knowing the mass of one component which reacted, the mass of the other component can then be determined by difference from the total mass of unknown mixture used. Finally, percent composition by mass can be calculated. In this experiment you will react a mixture of sodium nitrite, NaNO2, and sodium chloride, NaCl, of unknown composition with an excess of sulfamic acid, HSO3NH2. The NaCl is unreactive under these conditions, whereas the NaNO2 reacts to form nitrogen gas according to the following reaction equation. NOA(-,2)(aq) + HSO3NH2(aq) F( , )> HSOA(-,4)(aq) + H2O(l) + N2(g) You will collect the N2 gas at atmospheric pressure and room temperature and calculate from your data the moles of N2, moles of NaNO2, mass of NaNO2, mass of NaCl, and percent composition by mass of NaNO2 and NaCl in the mixture. Procedure Note that you will work in pairs for this experiment. An apparatus similar to that in the figure below will be available for each pair of students. As always, it is essential that safety goggles be worn while you are in the laboratory.  Apparatus for reaction of a mixture. Add sufficient water to the leveling bulb, raising it as appropriate, so that the measuring tube is filled to within 1 ml of the top calibration mark when the water level in the bulb is the same as in the measuring tube. Check the apparatus for leaks by inserting stopper A securely and lowering and raising the leveling bulb. If all the joints are tight, the level of the water in the measuring tube will return to its original level when the leveling bulb is raised to the same original level. Weigh 0.18 g of sulfamic acid, HSO3NH2, on a top loading balance, remove stopper A and add the sulfamic acid to the 50-ml Erlenmeyer flask. Also add 10. ml of distilled water. CAUTION: Sulfamic acid and sodium nitrite must never be mixed together as solids. In the presence of traces of water the solids react to evolve nitrogen and heat so rapidly as to be dangerous. Using an analytical balance, weigh just over 0.1 g of your unknown sample into a dry 1-dram vial. Record your masses in your lab notebook. Add 1 ml of distilled water to the vial, and lower the vial carefully into the Erlenmeyer flask. It should rest against the bottom and side of the Erlenmeyer flask without allowing any solution to either enter or leave the vial. Then insert stopper A securely into the Erlenmeyer flask. CAUTION: Be sure that the rubber tubing connecting the Erlenmeyer flask and the measuring tube is free of kinks. Finally, have your instructor check your apparatus. Before beginning the reaction, adjust the leveling bulb once again so that the water levels in the leveling bulb and the measuring tube are exactly the same. Then read the initial volume indicated by the meniscus in the measuring tube and record the initial volume in your lab notebook. Tip the 50-ml Erlenmeyer flask so that some mixing of the sulfamic acid solution with the solution inside the vial occurs. Gas evolution should begin. Over the course of the reaction, periodically lower the leveling bulb so that the water levels in the leveling bulb and measuring tube are approximately the same. This reduces the gas pressure in the system and helps prevent leaks. As gas evolution decreases, tip the Erlenmeyer flask more to achieve additional mixing. Continue to mix the solutions by gentle shaking and swirling of the flask until there is no further evolution of nitrogen gas. Then wait about 5 minutes to be certain that the contents of the flask have returned to room temperature. Adjust the leveling bulb so that water levels in the leveling bulb and the measuring tube are again exactly the same. At this point, the pressure of the gas is equal to atmospheric pressure. Read the final volume of gas in the measuring tube, and record it in your lab notebook. Also record in your lab notebook both the barometric pressure and room temperature. Clean the Erlenmeyer flask and vial carefully, and repeat the experiment twice more or until three successful runs have been completed. Be sure to use the same unknown number for all trials. Note that for each run the following data should be recorded in your lab notebook: unknown number, mass of unknown sample, initial volume of gas, final volume of gas, barometric pressure, and room temperature. Calculations: From the data for each run you will know the mass of the unknown sample and should be able to determine the volume of N2 gas collected. Determine the pressure of dry N2 gas by correcting for the vapor pressure of water at room temperature. Then calculate the moles of N2 collected, the moles of NaNO2 reacted, and the mass of NaNO2 reacted. By difference, determine the mass of NaCl in the unknown sample. Finally, determine the percent of NaNO2 and NaCl by mass in the unknown sample. From the results of three separate runs, calculate the average percent NaNO2 and NaCl by mass in your unknown sample. Report Your report should contain the following: 1. Partner's name. 2. Unknown number. 3. A table for each run containing the barometric pressure, room temperature, mass of the unknown sample, volume of N2 gas collected, vapor pressure of water, pressure of dry N2 gas, moles of N2 collected, moles of NaNO2 reacted, mass of NaNO2 reacted, mass percent NaNO2 in unknown sample, mass of NaCl in unknown sample, and mass percent of NaCl in unknown sample. 4. Sample calculations for one run to show how you obtained the numbers in item 3. 5. Average mass percent of NaNO2 and of NaCl in the unknown sample. Also answer the following "what if" questions, clearly showing your reasoning. 6. Would your calculated percent NaNO2 by mass be larger than, smaller than, or unchanged from the correct value as a result of the following mistakes? a. A leak occurred around stopper A in the mouth of the Erlenmeyer flask during evolution of nitrogen gas. b. You neglected to correct for the vapor pressure of water in your calculations. 7. Would your calculation of moles of nitrogen gas produced be larger than, smaller than, or unchanged from the correct volume as a result of the following mistakes? a. Twice as much sulfamic acid as required in the procedure was added. b. A high pressure system moved into the area very rapidly and the barometric pressure increased by 8 torr between the initial volume measurement and the final volume measurement, but you were unaware of this change. Bonus: Can the procedures in this experiment be used to determine the percent composition of a mixture of sodium nitrite and barium nitrite, Ba(NO2)2? Why or why not? Adapted from Chemistry in the Laboratory by C.W.J. Scaife and O.T. Beachley, Jr., Saunders, 1987. CH 141 Lab: Expt. VII -- -- CH 141 Lab: Expt. 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