Working with the Ideal Gas Law Prelab

Transcription

1 Working with the Ideal Gas Law Prelab NOTE: AT THIS POINT YOU WILL ANSWER ALL PRELAB QUESTIONS IN YOUR CARBON COPY LAB NOTEBOOK. BE SURE TO INCLUDE THE NAME OF THE EXPERIMENT AND THE QUESTIONS ASKED. AT THE START OF LAB YOU WILL HAND IN THE CARBON COPY, WHICH WILL CONTAIN ANSWERS TO ALL QUESTIONS EXCEPT QUESTION 3 WHICH SHOULD BE ON A SEPARATE PAGE SINCE YOU WILL NEED THIS FOR THE LAB SESSION. 1. What is the purpose of this experiment? 2. Show the calculation for the mass of NaNO 2 required to produce 40. ml of N 2 with a temperature of 25.6 o C and an atmospheric pressure of 739 mmhg. (use equation (2)) 3. Copy the results pages in your carbon copy notebook. 35

2 Working with the Ideal Gas Law This experiment will enable you to collect a gas (N 2 ) evolved in a given reaction and measure its temperature, volume and pressure. Assuming this is an ideal gas, the number of moles of nitrogen formed in this reaction can be calculated using the ideal gas equation. The amount of sodium nitrite, NaNO 2, in a sample will be determined by measuring the amount of nitrogen gas evolved in a chemical reaction. Introduction Nitrogen gas (N 2 ) is colorless and shows a great reluctance to react with other elements or compounds. This gas is a very useful material. The largest quantity of this gas is used to provide a nonoxidizing environment for packaged foods and wine. Nitrogen gas is also used to pressurize electric cables and telephone wires. The major use of this gas is in the production of fertilizers. Nitrogen occurs in protein and nucleic acids. Nitrogen constitutes about 3% of the human body. The nitrogen content of these biological materials is determined by adding excess nitrous acid (HNO 2 ) and a strong acid to the materials and measuring the amount of nitrogen gas evolved. In this experiment, an inorganic compound, sulfamic acid will be used as a nitrogen source instead of biological material. When sodium nitrite, NaNO 2, is dissolved in water, it dissociates into ions (Na +, NO 2 - ). Nitrous acid is a rather weak acid and when the sodium nitrite solution is combined with a solution of a strong acid like sulfamic acid, HNH 2 SO 3, almost all NO 2 - is converted to HNO 2 and the following reaction occurs releasing nitrogen gas: HNO 2 (aq) + NH 2 SO 3 - (aq) N 2 (g) + HSO 4 - (aq) + H 2 O (l) (1) Part of the nitrogen comes from HNO 2 and part from NH 2 SO 3 -. In this experiment, NH 2 SO 3 - is present in excess, so each mole of NaNO 2 you start with should lead to one mole of N 2 gas. Observe the reaction you will be working with in this experiment. NaNO 2 (aq) + HNH 2 SO 3 (aq) N 2 (g) + NaHSO 4 (aq) + H 2 O (l) (2) In Part I of this experiment, you will test this stoichiometric relationship from equation (2) with a known amount of sodium nitrite. In Part II, you will again use this relationship to determine the percentage of sodium nitrite in an unknown mixture. In order to calculate this percentage for Part II, you will first have to determine how many moles of nitrogen gas are produced in the reaction. We are going to assume ideality and use the following equation: n N2 ( Patm PH O )V 2 b = RT where the value of the constant R is ml atm/mole K and V b, is the volume change in the buret. P atm is the atmospheric pressure and PH 2 O is the vapor pressure of water at a certain temperature. A table of these vapor pressures at different temperatures are at the end of this 36

3 laboratory experiment. Be sure to estimate between temperatures when necessary. The temperature for this experiment will be the room temperature of the laboratory. Remember, when converting from mmhg to atm, the conversion is 1 atm = 760 mmhg. Procedure Part I 1. Using the ideal gas law equation, calculate the amount of NaNO 2 required to produce 40 ml of nitrogen gas (N 2 ) at the room temperature and pressure according to equation (2). Show this calculation on your calculations page. 2. Clean, dry and weigh a small test tube. Weigh approximately the amount of NaNO 2 you just calculated in step 1. Be accurate to 0.1 mg. Do not weigh out more than you need, if this happens, remove some of the NaNO 2 and reweigh. 3. Dissolve the NaNO 2 by adding 2 ml of distilled water to the small test tube. When the NaNO 2 is dissolved, place this test tube in a marked beaker to prevent it from overturning until you are ready to place it in the larger tube. 4. Fill a large beaker of distilled water and let it sit on your bench until it has reached room temperature (to within a degree). Be sure to record the room temperature since you will need this temperature to determine the vapor pressure of water. 5. Your apparatus for this experiment has already been set up for you and your instructor will inform you of the names of the different parts of the apparatus. Check that all of the connections on the apparatus are snug so there is no leakage and that the rubber tubing isn t pinched or folded shut. 6. You have to stabilize the pressures both inside and outside the system. This means both pressures must be the same. First, open the system by removing the rubber stopper from the large test tube. Support the leveling bulb at a level near the top of the buret and pour the room temperature water into the buret until it is nearly full. At this point, insert the rubber stopper back into the large test tube. By changing the height of the leveling bulb, you are changing the total pressure within the apparatus. When the water levels in the leveling the bulb and buret are identical, the pressures are identical both inside and outside the system. 7. To test for leaks, be sure all the stoppers are in place and raise the leveling bulb to maximum height. This increases the pressure inside the apparatus. Watch the water level inside the buret for a minute or so. If there is no change, then your apparatus is airtight. If the level rises slowly, there is a leak and you must find it. If the level stops rising when you pinch the rubber tubing connecting the large test tube to the buret, you have found the source of your leak. If the level continues to rise, the leak is in a connection near the test tube. When your apparatus is working correctly, return the leveling bulb to a level near the top of the buret. 37

4 8. Remove the large test tube from your apparatus. Weigh out 0.5g of sulfamic acid and place it in the large test tube. You do not need to be exact. Add 10 ml of distilled water to the large test tube and swirl to dissolve. 9. Carefully, lower the small test tube containing the nitrite sample into the large test tube. Do not allow the nitrite and the acid to come in contact. 10. Reconnect the large test tube to the apparatus and place it in the 1-litre beaker. At this time, carefully pour the room temperature water into the beaker until the level is above the liquid level inside the large test tube. 11. Recheck your apparatus for leaks. Next, repeat step 6 and have both liquid levels in the buret and leveling bulb identical. Make sure that when the levels are identical, the level is near the top of the buret. Keep the bulb and buret at this same level until the start of the experiment. 12. When everything is at the proper condition, place the rubber stopper back in the large test tube. 13. Record the initial volume in the buret. Remember that the large markings on the buret correspond to millilitres and the smaller markings to 0.1 millilitres. You initial volume should be recorded to the nearest 0.01 ml 14. One student should be keeping the levels in the buret and leveling bulb constant throughout step 15 while the other student performs step Lift the large test tube from the water bath and tip it so that a little of the nitrite solution flows out of the small tube into the sulfamic acid solution. Bubbling should occur. Return the large test tube to the water bath and shake it gently until the bubbling stops. Repeat this step until all of the nitrite solution has mixed with the acid. Keep shaking gently to insure good mixing. Avoid letting the solution from the test tube flow into the buret. 16. When the solutions from the small and large test tube mix, the gas is evolved and the liquid in the buret will drop quickly. Be sure to keep the two liquid levels about the same throughout the entire reaction. 17. When the reaction is complete, make sure the bath temperature water has remained the same. If not, make the proper adjustments. Adjust the leveling bulb to make the liquid levels identical and record the final volume reading on the buret. 18. Repeat this procedure one more time with another sample of sodium nitrite. 38

5 Part II 1. Obtain a packet containing NaNO 2 and a filler. You must determine the percent by mass of sodium nitrite in this mixture. Be sure to record the number for your unknown. 2. Weigh out about the same mass in your small test tube as you did in Part I. Fold your packet when you are finished weighing so that your unknown doesn t absorb any moisture. 3. Repeat the same procedure that you performed in Part I. Do this twice. 4. If you find there is inconsistency in your results, you may want to perform a third run and disregard the most inconsistent run. Should you do this, explain your reasons for doing so. 5. Hand in all of your carbon copies from this lab at the end of class. Vapor Pressure of Water at Different Temperatures Temperature ( o C) Vapor Pressure (mmhg) Temperature ( o C) Vapor Pressure (mmhg)

6 Results Part I Write the chemical reaction you will be studying in this experiment in the space below. Atmospheric pressure Room temperature Water vapor pressure Trial 1 Trial 2 Mass of small test tube and NaNO 2 Mass of empty small test tube Mass of NaNO 2 Moles of NaNO 2 Theoretical yield of moles of N 2 (eq (2)) Final Buret reading Initial Buret reading Volume change (V b ) P atm - PH 2 O Moles of N 2 gas produced experimentally % yield of N 2 Average % yield of N 2 Show a sample calculation for everything in your notebook. Include all conversions and units. 40

7 Part II Write the chemical reaction you will be studying in this experiment in the space below. Atmospheric pressure Room temperature Water vapor pressure Unknown # Trial 1 Trial 2 Mass of small test tube and unknown sample Mass of empty small test tube Mass of unknown sample Final Buret reading Initial Buret reading Volume change (V b ) P atm - PH 2 O Moles of N 2 gas produced Moles of NaNO 2 in unknown sample (assume 100% yield of N 2 ) Mass of NaNO 2 in unknown sample % NaNO 2 (by mass) in unknown sample Average % NaNO 2 in unknown sample Show a sample calculation for everything in your notebook. Include all conversions and units. 41

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