Chesapeake Campus Chemistry 111 Laboratory

Transcription

1 Chesapeake Campus Chemistry 111 Laboratory Objectives Calculate molar mass using the ideal gas law and laboratory data. Determine the identity of an unknown from a list of choices. Determine how sources of error affect your measurements. Introduction You may remember from unit one that all matter has mass and takes up space. It may be difficult to visualize a vapor occupying space or to feel the mass of the atmospheric gases pressing against you now. This laboratory takes advantage of several properties of gases including their mass and ability to occupy the entire volume of a container to allow the calculation of the molar mass for an unknown compound. The gas laws in discussed in class relate other properties of gases. Boyles law demonstrates how pressure and volume of a gas are inversely related at constant T and n. Charles Law shows how temperature and volume of a gas are directly proportional when pressure and n are constant. Avogadro s Law shows how all gasses occupy the same volume per mol (or number of particles). This means that regardless of the identity the compound, all gas samples of the same number of mol occupy the same volume. These concepts are combined in the ideal gas law which states: PV = nrt Where P is pressure in atm, V is volume in L, n is the mols of the gas, R is the gas constant ( Latm/molK), and T is the temperature in Kelvin. This equation can be rearranged to calculate any variable. In this lab you are interested in finding the mol of an unknown sample so we can rearrange the equation to find n by n = PV RT Ideal gases should behave ideally. This means that we assume the gas will occupy the entire volume of a container. The particles will be moving rapidly, randomly and constantly. Any collisions between gas particles are elastic leaving kinetic energy intact. During this lab you will be measuring out several ml of an unknown volatile compound. You will add this small volume to a flask and then cover it loosely with foil. Poking a few holes in the foil will allow some of the compound to leave as it is heated. When the flask is heated, the volatile liquid will vaporize and occupy the entire space of the flask. Because gases occupy MUCH larger volumes than liquids, the volume of liquid you measure out will be more than sufficient to occupy the whole flask once it is vaporized. The pressure of the gas in the flask will be the much higher than the pressure of the gas in the laboratory at first. However as you heat the sample, some of the rapidly moving particles will eventually leave the flask through opening at the top of the flask. This will continue to happen until the pressure in the laboratory and the flask are the same (when all liquid has been vaporized). You can use the temperature you heated the flask to, the pressure of the laboratory, the volume of the flask and the gas constant to find the mol of the unknown compound you are using. At the end of the experiment when you cool down the compound, you can also get the mass of the liquid in grams. This will allow you to calculate the molar mass (g/mol) of the unknown compound. Using a list of possible choices (listed in Table 1), determine the identity of your unknown. Tidewater Community College. CC-BY 2014 Page 1

2 Possible Unknown Chemical Formula Molar Mass (g/mol) Methane CH Ethane C2H Methanol CH4O Ethanol C2H6O Acetone C3H6O Benzene C6H Ethyl Acetoacetate C6H10O Table 1: Possible Unknowns Used in Lab Materials Student tray containing the following: o Aluminum foil o Rubber bands o Scissors o Needle or paper clip o boiling stones o 125 ml Erlenmeyer flask o 100 ml graduated cylinder o 600 ml beaker o 10 ml graduated cylinder o Thermometer o tripod o watch glass o Container of Unknown Liquid A Balance Safety and Notes It is important to follow the safety guidelines below while performing this lab All compounds should be treated with caution. Inhalation of the vapors should be avoided. Skin contact should be limited. Spills should be dispersed on paper toweling and the towel removed to evaporation in the fume hood. The unknown is a flammable liquid, use caution when it is being heated. All chemicals should be disposed of in the CHM 111 Waste container in the back hood. Wash, clean and put back all equipment before leaving lab. Experimental Procedure and Data 1. Set up the ring stand, ring, tripod and hot plate as shown in the diagram. 2. Clamp a thermometer to the ring stand to measure the temperature in the middle of the solution. 3. Fill a 600 ml beaker with water and add a few boiling stones. 4. Begin heating water. 5. Obtain a clean, dry 125 ml Erlenmeyer flask 6. Obtain a piece of aluminum foil and rubber band to serve as a cover for the experiment (but do not attach yet. Tidewater Community College. CC-BY 2014 Page 2

3 7. Obtain the mass of the flask, foil and rubber band. 8. Measure 4 ml of the unknown liquid. 9. Pour the liquid into the 125 ml flask 10. Fashion the foil tightly over the top of the flask with the rubber band. 11. Punch a several small holes in the foil using a needle or paper clip. 12. Immerse the Erlenmeyer in the water. The water level should be high enough to cover most of the flask but not so high as to allow water to enter through around neck near the foil. 13. Clamp the flask using a second clamp to prevent the flask from bobbing up and down. 14. Heat the water to about 85 C. You do not want the water to boil as that could cause splashing onto the top of the foil. Maintain this temperature (+/- 10 degrees). 15. When the liquid in the flask has evaporated, you can move to the next step. (Usually this takes ~ 2-5 minutes but may take longer if the water bath was cool when you started. 16. Remove the flask from the water and allow the flask to cool. Turn the heat source off. 17. Wipe the outside of the flask to completely remove water being especially careful about drying the top edges. 18. While the flask is cooling, obtain the pressure of the laboratory from the barometer at the front of the room. 19. Only once the flask has cooled, obtain the mass of the flask set-up and record your data. 20. Repeat this process (steps 5-19) for at least two trials. Determine the molar mass from separate trials. 21. After the final trial remove the cover assembly and rinse the flask thoroughly. 22. Use a graduated cylinder to fill the flask with water. Calculate the amount of water needed to fill the flask completely to the top (to the same volume the gas occupied). This may take several graduated cylinders of water. Be sure to add the volumes to the correct number of significant figures. Tidewater Community College. CC-BY 2014 Page 3

4 Name Lab Partner Name Date Bin # Pre-Lab Questions Note This pre-lab must be completed before you come to lab. 1. Write the ideal gas law. Give the units for each variable. 2. A laboratory group measures an excess of an unknown liquid. They add this to an empty flask for which they calculate the volume. They heat the sample n a water bath until all liquid has been vaporized for several minutes. The sample is cooled and measured. Use their laboratory data (listed below) to answer the following questions: Flask Volume ml = m L. Mass of flask g. Mass of Flask + volatile liquid g. Mass of volatile liquid 3.44 g. Temperature of water bath ( C) 84.1 C. Pressure in lab (atm) atm. Gas Constant (L*atm) /( mol*k). a. Calculate the mol of gas in the flask using the ideal gas law: b. Calculate the molar mass of the unknown: 3. If a student added more than 4 ml of volatile liquid, would it affect the laboratory today? How? What if less liquid was added? Tidewater Community College. CC-BY 2014 Page 4

5 Name Lab Partner Name Date Bin # Results, Discussions and Post Lab Questions Unknown ID Mass of empty flask, foil and wire Trial 1 Trial 2 Mass of flask, foil, wire, and volatile liquid AFTER heating Mass of volatile liquid Pressure of Room (atm) Temperature of water bath ( C) Temperature of water bath (K) Volume of flask (if more than one graduated cylinder of water added, add the volumes to obtain the final) Mol of gas calculated from ideal gas law Molar mass of gas Average Molar Mass of unknown Tidewater Community College. CC-BY 2014 Page 5

6 Name Lab Partner Name Date Bin # Post Lab Questions 1. Based on the molar mass you calculated for your unknown, what possible unknown (listed in Table 1) did you have? Calculate the percent error in the molar mass of your unknown based on this substance. 2. What were some sources of error in your experiment and how did they affect your measurements? (For full credit list at least 2 sources of error as well as how each affected the molar mass value you calculated.) 3. What would have been a more accurate way to perform today s experiment? Tidewater Community College. CC-BY 2014 Page 6