Analysis of a KClO3 Mixture and Determination of “R”, Slides of Law

Download Analysis of a KClO3 Mixture and Determination of “R” and more Slides Law in PDF only on Docsity! 11-1 Experiment 11 Analysis of a KClO3 Mixture and Determination of “R” Pre-Lab Assignment Before coming to lab: • Read the lab thoroughly. • Answer the pre-lab questions that appear at the end of this lab exercise. Purpose The mass percent of potassium chlorate in a mixture will be determined by its thermal decomposition and production of oxygen gas, which will be measured via displacement of water. The Ideal Gas Constant (R) and the molar volume of an ideal gas at standard temperature and pressure (STP) will also be determined by measuring the conditions of the oxygen gas produced. Background When potassium chlorate (KClO3) is heated, it thermally decomposes to produce solid potassium chloride and oxygen gas by the following balanced reaction in Eqn. 1. 2 KClO3(s) → 2 KCl(s) + 3 O2(g) Eqn. 1 Normally this reaction requires temperatures of 400°C, but a catalyst such as manganese dioxide (MnO2) can be added to lower the temperature to a more achievable 250°C. Since the KCl and MnO2 are solids, they will be left behind in the reaction container while O2(g) can be driven off to be collected and measured separately. Since gases are hard to measure in a laboratory, the O2(g) will instead be measured by the displacement of water so that the volume of water collected is equal to the volume of O2(g) produced. However, since water is a liquid, the gas trapped inside the container will be a mixture of O2(g) and H2O(g), or water vapor, so that the total pressure will be the sum of both gases. The Ideal Gas Law relates the pressure (P), volume (V), temperature (T), and moles (n) for any gas in terms of the Ideal Gas Constant, R, as seen in Eqn. 2: R = PV nT Eqn. 2 R has a standardized value of 0.082057 L*atm/mol*K. By measuring the P, V, T, and n of any one gas, R can be determined and should be close to the accepted value. The molar volume for an ideal gas is defined as the L/mol that it occupies at 273.15 K and 1.00 atm, or standard temperature and pressure (STP). With rearranging, it can be expressed via the Ideal Gas Law as well as in Eqn. 3: V n = RT P Eqn. 3 This also has a standardized value of 22.414 L/mol at STP. 11-2 For this experiment, an unknown mixture of KClO3, KCl, and MnO2 solids are heated to allow just KClO3 to decompose and the O2(g) made allowed to escape into a second vessel. The change in mass before and after the reaction will correspond to the mass of O2(g) produced since all other components are solids. The amount of O2(g) produced will also be used to determine the mass percent of KClO3 in the original sample. The pressure, temperature, and volume of O2(g) produced will also be measured. The pressure inside the container the sum of both gases, O2(g) and H2O(g) as seen in Eqn. 4. To find the pressure of O2(g) alone, the tabulated vapor pressure of water at the gases’ temperature must be subtracted from the total. Ptotal = PO2 + PH2O Eqn. 4 The total pressure of the container is assumed to be equal to the outer atmospheric pressure for the day once corrected for the expansion of mercury inside the barometer by using Eqn. 5. P(atmosphere) – (1/8)(T of barometer, in °C) = P(corrected) Eqn. 5 The temperature of O2(g) can be measured directly with a thermometer and the volume of O2(g) will be measured via the water it displaces. The value for R and the molar volume at STP can then be calculated and compared to standardized values. Example Problem: Determination of the Mass Percent of KClO3 A 1.565 g mixture of KClO3, KCl, and MnO2 is heated and the O2(g) produced allowed to escape to displace water. At the end of the reaction, the remaining mixture weighed 1.323 g. Calculate the mass percent of KClO3 present in the original mixture. Step 1: Find the mass of O2(g) produced 1.565 g before reaction – 1.323 g after reaction = 0.242 g O2 made Step 2: Find the mass KClO3 in the original sample 0.242 g O2 × 1 mol O2 32.00 g O2 × 2 mols KClO3 3 mols O2 × 122.55 g KClO3 1 mol KClO3 = 0.617 g KClO3 Step 3: Find the mass percent KClO3 in the original sample 0.617 g KClO3 1.565 g mixture × 100 = 39.4 % KClO3 11-5 6. Raise the beaker until the Florence flask is full again as in Step 4. Clamp the tubing. Stopper the ignition tube and dump out the water from the 600 mL beaker. Do not dry it. 7. Unclamp the tubing. A small amount of water should flow into the beaker and then stop. Note: if the flow of water to the beaker does not stop, your apparatus has a leak. Tighten all stoppers and check all tubing and repeat Steps 3-7 as necessary. 8. Using a Bunsen burner directly under the solid sample in the ignition tube, gently begin heating. Some of the solids will begin to liquefy and produce white vapors. As the latter subsides, begin increasing the heat to ensure all the KClO3 has fully reacted. The volume of water in the 600 mL should steadily increase so long as O2(g) is still being produced. 9. Continue heating until no further vapors or change in volume of water in the beaker is observed and then heat for five additional minutes. The mixture in the ignition tube should be entirely white solid; no purple color or molten appearance should remain. Turn off the Bunsen burner. Note: do not melt the stopper or the rubber clamp holding the ignition tube. 10. Allow the apparatus to fully cool to room temperature while still stoppered. 11. Clamp the tubing between the Florence flask and 600 mL beaker. Now both stoppers can be removed and the apparatus disassembled. 12. Use a thermometer to measure the temperature (TO2) of the gases inside the Florence flask, not the water. 13. Use a graduated cylinder to measure the volume of water (VO2) inside the 600 mL beaker. Note: you may have to use the cylinder repeatedly by emptying it between portions. 14. Weigh the ignition tube and same beaker used in Step 1. Record this weight in your data sheet. 15. Record the atmospheric pressure (Patm) and the temperature of the barometer (Tbarometer) for the day from the classroom barometer. 16. The residue inside the ignition tube can be loosened with water and disposed of in the labeled waste container. 17. Repeat Steps 1-16 for a second trial. 18. Calculate the mass percent KClO3, R, and the molar volume at STP for each trial. These values should agree with each other as well as the standardized values for R and molar volume with reasonable percent error. 11-6 11-7 Experiment 11—Data Sheet Name: ________________________________________ Unknown #: ___________________________________ Trial One Trial Two 1. Mass of ignition tube and beaker (g) ___________ ___________ 2. Mass of ignition tube, beaker, and sample ___________ ___________ before reaction (g) 3. Temperature of O2(g) (TO2, °C) ___________ ___________ 4. Volume of O2(g) (VO2, mL) ___________ ___________ 5. Mass of ignition tube, beaker, and sample ___________ ___________ after reaction (g) 6. Atmospheric Pressure (Patm, mmHg) ___________ ___________ 7. Temperature of barometer (Tbarometer, °C) ___________ ___________ 8. Mass of sample (g) ___________ ___________ show calculation: 9. Mass of O2(g) produced (g) ___________ ___________ show calculation: 10. Mass of KClO3 in sample (g) ___________ ___________ show calculation: 11. Mass Percent of KClO3 in sample (%) ___________ ___________ show calculation: 12. Average Mass Percent of KClO3 (%) ___________ show calculation: 11-10 11-11 Trial One Trial Two 19. Pressure of O2(g) (PO2, atm) ___________ ___________ show calculation: 20. Gas Constant, R (L atm/mol K) ___________ ___________ show calculation: 21. Percent Error for R ___________ ___________ show calculation: 22. Molar Volume at STP (L/mol) ___________ ___________ show calculation: 11-12 11-15 4. A student failed to correct the atmospheric pressure for the expansion of mercury, reporting the uncorrected pressure as the total pressure for their calculations. Will this error change the student’s calculated values? If so, will the change be higher or lower than their true values? For each, explain your reasoning clearly. a. Mass Percent KClO3 b. Gas Constant, R c. Molar Volume at STP 5. What mass of KClO3 (in kg) must be decomposed to supply 8 people with enough oxygen for one day on a small submarine? According to NASA, the average person needs about 0.84 kg of O2(g) per day. 2 KClO3(s) → 2 KCl(s) + 3 O2(g) 11-16 11-17 Experiment 11—Pre-Lab Assignment Name: ________________________________________ For all calculations, show all work and draw a box around the final answers. 1. Write the balanced equation for the thermal decomposition of potassium chlorate. 2. Why is it important that you NOT heat the potassium chlorate in contact with the rubber stopper? 3. What data items are used in this experiment to determine the following calculated values: a. Mass Percent of KClO3 b. Gas Constant, R c. Molar Volume