Molar Mass by Vapor Density Formal Lab Report

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Jan 9, 2024

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Molar Mass by Vapor Density Purpose : The purpose of this experiment is to determine the molecular weight of a volatile liquid using the Dumas Method. Theory :

When using the Dumas method we will be able to weigh the vapor that is left in our flasks after the experiment. With that value we could then solve for the molecular weights of the unknowns by using the ideal gas law: 1. PV = nRT With our lab manual we know that the P represents pressure, V is for volume, T stands for the temperature, n is the number of moles of gas, and R is the gas law constant of 0.08206 atm(L) / mol(K). With the Dumas method we are able to substitute n for grams or the molecular weight of the unknown. We are then left with the equation g / MW x RT. If we rearrange that equation we end up with: 2. MW = mRT/VP The equation above shows how we would also need to figure out the mass of the unknown acid (m) because Equation 1 already includes the rest of the information needed in order to solve for our molecular weight. Procedure : 1. Weigh a 125 mL Erlenmeyer Flask with a rubber septum on an analytical balance. 2. Dry septum, flask, and clean then record mass. 3. Obtain about 3-4 mL of unknown. Pour into the flask then place septum over top. Place a small piece of capillary tubing through the puncture in the septum to allow air and excess vapor to escape when the flask is heated. 4. Clamp flask so it is up to its neck in a large beaker (400-600 mL) about half to 3 quarters full of water. 5. Add 3 or 4 booking stones to the water in the beaker. 6. Heat water to a boil and boil for 2 minutes. After all liquid disappears from flask, do not remove the flask yet.

7. Remove and dry outside of the flask and edges around the septum. Weigh flask, septum, and condensed liquid immediately. Record mass. 8. Repeat steps above with the same unknown. 9. Record barometric pressure and the temperature to which the flask was heated. 10. Fill the flask with water. Weigh flask, septum, and water on triple beam balance. Record mass. 11. Calculate molecular weight of the unknown. Data : Mass of Empty Flask 1 w/ Rubber Septum (g) 100.5101g Mass of Empty Flask 2 w/ Rubber Septum (g) 96.2093g Table 1: Recorded Mass of Empty Flasks with Septum Mass of H2O in Flask 1 137.8699g Mass of H2O in Flask 2 137.1507g Table 2: H2O Mass in Flasks Temperature of H2O in Flask 1 (C) 22.8 C Temperature of H2O in Flask 2 (C) 22.1 C Average Temperature of H2O (C) 22.4 C Table 3: Recorded Temperatures of H2O in Flask 1 and 2 and Average Mass of Empty Flask 1 + Rubber Septum + Unknown (g) 101.0276g Mass of Empty Flask 2 + Rubber Septum + Unknown (g) 96.5423g Table 4: Total Volume of Mass of Empty Flasks + Rubber Septum + Unknown Unknown Mass in Flask 1 (g) 0.5175 g Unknown Mass in Flask 2 (g) 0.333 g Table 5: Calculated Mass of Unknown #20 Temperature of H2O (K) 295.4 K

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Pressure (atm) 0.9908 atm Table 6: H2O Temperature and Pressure Conversions Flask 1 MW 91.75 g/mol Flask 2 MW 59.38 g/mol Table 7: Molecular Weight of Unknown #20 in Trial 1 and 2 Results and Discussion : Table 2 Calculations for Mass of H2O Flask 1: 238.38 g - 100.5101 g = 137.8699g Flask 2: 233.33 g - 96.2093 g = 137.1507g Table 3 Calculation for Average Temperature Average Temperature of H2O: = 22.8 C + 22.1 C / 2 = 22.4 C Table 4 Calculations For Unknown in Each Flask Flask 1: 101.0276 g - 100.5101 g = 0.5175 g Flask 2: 96.5423 g - 96.2093 g = 0.333 g Table 5 Calculations for H2O Temperature and Pressure H2O Temperature in Kelvin: = 22.4 C + 273 = 295.4 K Pressure in atm: = 753 mmHg = 1 atm / 760mmHg = 0.9908 atm Table 6 Calculations for Molecular Weight

MW for Trial 1: 0.5175𝑔 𝑥 0.08206 𝑎𝑡?(?) ?𝑜?(?) 𝑥 295.4? 0.9908𝑎𝑡? 𝑥 0.1380? = (12.5445g/mol) / 0.1370 = 91.75 g/mol MW for Trial 2: 0.333𝑔 𝑥 0.08206 𝑎𝑡?(?) ?𝑜?(?) 𝑥 295.4? 0.9908𝑎𝑡? 𝑥 0.1372? = (8.0721g/mol) / 0.1370 = 59.38 g/mol Using the Dumas method helped us determine vapor density in order to determine the molecular weight of a vaporized unknown liquid with Equation 2. We were to first collect the weights of an empty 125 mL flask + rubber septum ( Table 1 ), and mass of the flask + rubber septum + condensed unknown solution ( Table 5 ). We recorded the room temperature water shown in Table 3 and in Table 6 we recorded the barometric pressure in the room and converted it to atm. In order to find our values in Table 2 we had to subtract the Unknown Mass in the Flasks from the Mass of Empty Flask 1 w/ Rubber Septum (g). Although the procedure was followed, there were some mistakes that took place during the lab that we did not notice until the end. A couple of errors that may have happened in the experiment include boiling the flask for too long. We did not notice that the liquid had already disappeared so we let the flask stay in the boiling water for longer than it should’ve been. This caused the mass of our unknown to become higher than it actually was. This value alone affected our entire outcome when finding our molecular weight. Trial 2 had little to no error so we were luckily able to compare the two and realize that trial 1 had some errors. Thinking of these errors in the future can keep us from repeating the same mistake within similar experiments. Conclusion : The purpose of this experiment was to determine the molecular weight of a volatile liquid using the Dumas Method. Based on our results, it was clear to see that it is possible to

determine the molecular weight and ultimately figure out what our unknown was. To do this we had to collect information such as the mass of our unknown, the pressure in the room, the total volume the flask could hold, and the temperature of the room. A majority of these values also had to be converted before we could plug them into Equation 2 . After the conversions and plugging in we got 91.75 g/mol for our first trial and 59.38 g/mol for our second trial. There were a couple errors while conducting the experiment for trial 1, but trial 2 was accurate. With the molecular weight of 59.38 g/mol we looked back and concluded that our unknown was acetone (58.0 g/mol). References : Goldwhite, H., Tikkanen, W., Kubo-Anderson, V., Mathias, E., and Jalali Heravi, M.. Experiment 07: Molar Mass by Vapor Density. Experiments in General Chemistry . 6th Edition. (47-49)

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