Molar Mass of a Volatile Liquid

I NFORMAL LAB REPORT, EXPERIMENT 12 “MOLAR MASS OF A VOLATILE LIQUID” DISCUSSION: This lab focuses on the molar mass of a volatile liquid. The goals include measuring the physical properties of pressure, volume, taking temperature for a gaseous substance, and determining the molecular weight of a volatile liquid. Basically, the experiment’s main focus is to determine the molecular weight of that volatile liquid after it has been vaporized and using the barometer to record the pressure of the condensed vapor (Dumas method). This experiment, like many, requires one to be persistent with all the details that are given so the final results can come out to be as expected. First, an Erlenmeyer flask is obtained, cleaned, dried, and properly weighed on a scale to the nearest 0.001 gram. It is important to remove any remaining water particles since this will be what provides the experiment’s accuracy. A piece of aluminum foil was used to cover the flask and a rubber band was used to secure it, these items are weighed as well. Next, the unknown liquid is measured and 5 mL of that liquid is poured into the flask and the aluminum then covers the flask again but this time holes are pierced among the surface. A water bath is prepped and boiling chips, an insoluble porous stone to make substances boil more calmly, is added and the water bath is placed on the bunsen burner. Next, the dry Erlenmeyer flask is held with a utility clamp and submerged into the water bath (the flask/clamp must nottouch the beaker). A moderate flame is used to the sample to heat up to the temperature of the boiling water. The sample is then removed from the water bath, cooled, then measured. It is important to note that the remaining vapor in the flask may condense. This process is then repeated twice. Next, the volume and pressure of the vapor are determined. In order to find the volume of vapor, the flask is filled with water to the top and the volume is measured to 0.1 mL. In order to find the atmospheric pressure of the vapor present inside the flask, the barometer is used so all the atmospheric pressure in the atmospheres is known. Generally, the vapor mass is found from the difference between the empty vessel and the vapor-filled vessel. In getting the results for this experiment many formula were used in the process. Here we alternate with using the ideal gas law, P V = n RT .For part A, the first thing done was finding the mass difference between the empty vessel and the vapor-filled vessel. This is done by taking the difference of the mass of the flask and the vapor and subtracting it from the mass of the flask. To find the moles of the vaporized liquid, n is isolated. P is then isolated to find the barometric pressure pressure. Finally the molar mass of the compound is calculate. The formulas used for each step for the three trials are shown bellow: 1. mV APOR = m FLASK + V APOR − mFLASK 2. nV APOR = 3. P = PV RT nRT V 4. M compound = mvapor nvapor There is no theoretical value in the experiment, therefore, no percent error is calculated. However, it is known that gases with large intermolecular forces and large volumes do not behave according to the gas law equation, leading the gases to cause deviations. It was found that the moles of the vaporized liquid are somewhat constant and do not differ from each other very much. From there, the average molar mass was calculated. This experiment can be used to determine the molar mass of any volatile liquid. This experiment focused on the determination of physical properties of components such as pressure, volume, and temperature for the gaseous substance. This experiment also determines the molar mass of the volatile liquid by using the ideal gas law in many ways. CALCULATIONS AND RESULTS: Part A Trial 1 Trial 2 Trial Mvapor 0.3125 g 0.2854 g 0.3034 g n= PV/RT 0.0093 mol 0.0094 mol 0.0094 mol 0.992 atm 1 atm 1 atm 33.6 g/mol 30.4 g/mol 32.3 g/mol P=nRT/v M compound = mvapor nvapor Average molar mass = 32 g/mol Part B Carbon Hydrogen Oxygen Mass in the original sample 1875 g 0.0625 g 0.2496 g Moles combusted 0.0156 mol 0.0624 mol 0.0156 mol Empirical formula: C H 4 O WORK: ERRORS: Many systematic errors can occur in any experiment. These systematic errors will eventually give the experiment’s results with inaccurate data. For example, in the beginning, if the Erlenmeyer flask is not dried all the way, the water particles that get leftover will alter the mass significantly for the end results. Another systematic error could be not poking the holes for the aluminum small enough. If the holes are too big, too much of the gas will outflow These holes will ensure that no water gets left behind. Finally, if the liquid is not boiled gently then it may inflame. This is because most unknowns given are combustible. POST-LAB QUESTIONS: 1. Part A.1 a. The mass of vapor in the flask will be reported to be too high since there are water molecules still present outside the flask. b. The molar mass reported would also be too high because it is now considered to be part of the vapor’s molecular weight. 3. Part A.2 c. The mass of vapor would be reported to be low because the water vapor will have effused. d. The molar mass will be reported low as well because the mass of the vapor is low’ 5. Part B.2 e. The molar mass of the vapor will end up being low because mass and temperature are related to each other. 7. Part C.2. f. Molar mass is inversely proportional to Pressure. That means, the pressure is high and molar mass is low