The Dumas Method
Wiley Turner
Liam Doyle, and Liam Springer
The Packer Collegiate Institute, 170 Joralemon Street, Brooklyn, New York, 11201
Performed on December 5th and 6th, 2017
Due on December 21st, 2017
Liam Doyle, and Liam Springer
The Packer Collegiate Institute, 170 Joralemon Street, Brooklyn, New York, 11201
Performed on December 5th and 6th, 2017
Due on December 21st, 2017
Objective:
The objective of this lab was to determine the molar mass of an unknown, volatile liquid using the Dumas Method and the Ideal Gas Law.
Post-Lab Questions:
1a.
Depending on how much liquid was not vaporized before the tip was sealed, the results would change. If there was leftover liquid in the flask that had not been evaporated into gas, the mass of the condensed liquid would seem bigger than it actually is due to the fact that not all of the liquid was from the condensation of the gas. This increased mass would make our molar mass higher. In addition, if there was not enough of the volatile liquid vaporized to push out all the air, we would find that less water occupied the flask upon the vacuum being broken, thus leading us to find a higher molar mass for the substance.
1b.
If the flask wasn’t left in the boiling water bath for long enough to reach thermal equilibrium with the water, we would find that our calculated molarity of the volatile substance would be lower than it actually was. This is because when plugging the incorrect, higher temperature of the water into the Ideal Gas Law, the number of moles of substance would decrease, thus leading us to believe that the molar mass of our substance was higher than in reality.
1c.
If the inside of the flask contained drops of water during the initial massing, we would record a lower mass of the condensed unknown substance, through the subtraction of the different apparatuses. Due to this change in mass, we would find that the molar mass would decrease as there would be less grams for the same amount of moles as before.
1d.
If the flask was not thoroughly dried for the final massing, we would record that there was a higher mass of the volatile, condensed liquid as the water on the outside of the flask would be counted towards the mass of the unknown substance. This increase in mass would lead us to believe that the molar mass of the substance would be higher than it actually was as it would be more grams per mole of the substance.
1e.
If the flask was not completely sealed, the pressures would normalize and when cracking the flask under the water, we would find that no water would enter the flask. This would make our data useless as we would not be able to find the volume that the gas of the volatile liquid took up in the flask, thus not allowing us to find the molar mass of the substance.
2.
There are many limitations for using the boiling water as a bath. For one, we were not able to tell if the flask had reached thermal equilibrium, thus possibly leading us to input an incorrect temperature value into our Ideal Gas Law equation. This could be fixed by allowing the water and flask to heat up together as to make sure that they are at the same temperature. Another limitation would be if the liquid had a higher boiling point than water. If the liquid were to have a higher boiling point, we would find that it wouldn’t actually evaporate and none of the air in the flask would be pushed out. This could be fixed by using a liquid with a higher boiling point than water to create the bath. One example would be to use salted water which has a higher boiling point than pure water.
3.
We found that our theoretical volume gave us more accurate results. This is due to the fact that our theoretical volume represents the amount of gas from the volatile liquid that was in the flask before it was taken out of the boiling water and a slight amount of air was let in. This more accurate volume would thus correlate to more accurate results. The experimental volume from the pressure-equalization method was used to determine how accurate the recorded value for the mass of the condensed vapor was. By using this method, we are able to determine how much of the flask was filled up with gas and how much was filled up with air. When massing the condensed vapor, the air that was able to snake in before the tip was flame sealed, makes a difference, while ever so slightly, in mass. By gauging how much air was in our flask compared to the gas, we are then able to demonstrate how accurate the mass of the vapor was.
The objective of this lab was to determine the molar mass of an unknown, volatile liquid using the Dumas Method and the Ideal Gas Law.
Post-Lab Questions:
1a.
Depending on how much liquid was not vaporized before the tip was sealed, the results would change. If there was leftover liquid in the flask that had not been evaporated into gas, the mass of the condensed liquid would seem bigger than it actually is due to the fact that not all of the liquid was from the condensation of the gas. This increased mass would make our molar mass higher. In addition, if there was not enough of the volatile liquid vaporized to push out all the air, we would find that less water occupied the flask upon the vacuum being broken, thus leading us to find a higher molar mass for the substance.
1b.
If the flask wasn’t left in the boiling water bath for long enough to reach thermal equilibrium with the water, we would find that our calculated molarity of the volatile substance would be lower than it actually was. This is because when plugging the incorrect, higher temperature of the water into the Ideal Gas Law, the number of moles of substance would decrease, thus leading us to believe that the molar mass of our substance was higher than in reality.
1c.
If the inside of the flask contained drops of water during the initial massing, we would record a lower mass of the condensed unknown substance, through the subtraction of the different apparatuses. Due to this change in mass, we would find that the molar mass would decrease as there would be less grams for the same amount of moles as before.
1d.
If the flask was not thoroughly dried for the final massing, we would record that there was a higher mass of the volatile, condensed liquid as the water on the outside of the flask would be counted towards the mass of the unknown substance. This increase in mass would lead us to believe that the molar mass of the substance would be higher than it actually was as it would be more grams per mole of the substance.
1e.
If the flask was not completely sealed, the pressures would normalize and when cracking the flask under the water, we would find that no water would enter the flask. This would make our data useless as we would not be able to find the volume that the gas of the volatile liquid took up in the flask, thus not allowing us to find the molar mass of the substance.
2.
There are many limitations for using the boiling water as a bath. For one, we were not able to tell if the flask had reached thermal equilibrium, thus possibly leading us to input an incorrect temperature value into our Ideal Gas Law equation. This could be fixed by allowing the water and flask to heat up together as to make sure that they are at the same temperature. Another limitation would be if the liquid had a higher boiling point than water. If the liquid were to have a higher boiling point, we would find that it wouldn’t actually evaporate and none of the air in the flask would be pushed out. This could be fixed by using a liquid with a higher boiling point than water to create the bath. One example would be to use salted water which has a higher boiling point than pure water.
3.
We found that our theoretical volume gave us more accurate results. This is due to the fact that our theoretical volume represents the amount of gas from the volatile liquid that was in the flask before it was taken out of the boiling water and a slight amount of air was let in. This more accurate volume would thus correlate to more accurate results. The experimental volume from the pressure-equalization method was used to determine how accurate the recorded value for the mass of the condensed vapor was. By using this method, we are able to determine how much of the flask was filled up with gas and how much was filled up with air. When massing the condensed vapor, the air that was able to snake in before the tip was flame sealed, makes a difference, while ever so slightly, in mass. By gauging how much air was in our flask compared to the gas, we are then able to demonstrate how accurate the mass of the vapor was.
Conclusion:
Overall, we were fairly successful in using the Dumas Method to determine the molar mass of the unknown volatile liquid. In trial 1, we found that from the sealing of the tube, 1.048 grams of condensed liquid formed and that the gas had taken up 271 mL of the flask from the volume that the water filled. The experiment was performed at a pressure of 761.5 torr and a boiling water temperature of 269.0 K. From this data, we were able to use the Ideal Gas Law to determine that the molar mass of the volatile liquid was 116 grams per mole. While this number is relatively close and isn’t an atrocious value, we still had a 61.9% error when our experimental molar mass was compared to the actual molar mass of 2-butanol (72.104 grams per mole). Though our first trial was semi-successful, our second trial was atrocious as we were not able to record data for the volume of the gas. This is because, in the process of sealing, the tip did not completely close leading to the pressures normalizing and the gas not being able to condense and form the partial vacuum needed to suck up the water. Because of this error in our second trial, we only had one data point to work with which limited the amount of analysis that we could have done.
Even in our first trial, there were many errors that could have affected the experiment. For example, when forming the special glassware tip, we may have made the curve too sharp so that the vapor of the volatile gas is unable to actually escape and push the air out. This error is easily fixable as we could have taken more time to make a perfect tube which would then give us more accurate results as the air would be more readily pushed out through the more slight curve than the sharper curve and there would be far less air in our partial vacuum. Another error was the fact that after taking the flask out of the boiling water and before sealing the tip, air may have leaked in through the tip, causing the partial vacuum to be even more partial and thus allowing less water to flood the flask, giving us an inaccurate value for the volume of the gas. While there is a relatively easy way of resolving this, it does come with a small amount of danger. In theory, this could be solved by sealing the tip while the flask is still in the water. However, the sudden pressure difference between the inside and outside of the flask may cause the flask to explode. Another way to resolve this would be to use a smaller bunsen burner that could be closer to the heating plate, thus lowering the time that the open tip of the flask is exposed to the air. A third error was the fact that we may have heated the volatile liquid for too short of a time. It was very hard to tell when the liquid had fully evaporated even when pulling the flask out or to even know if the flask had reached thermal equilibrium. If the liquid had not fully evaporated, it may be possible that not all the air in the flask was pushed out as there was not enough vapor to push it out. If the flask hadn’t reached thermal equilibrium with the water, the temperature value of the water would not be correct when plugged into the Ideal Gas Law and would yield an incorrect molar mass. This is easily fixable by allowing the flask to heat for longer and to more occasionally check the liquid level inside the flask to make sure it has all evaporated before removing it.
In addition to fixing the errors of our experiment, there are also other ways to improve it. For example, if we had more time, we could conduct more trials and be able to perfect our glassware. The extra trials and the better glassware would also yield more accurate results. In addition to this, we could also conduct trials with different compounds of other molar masses. This would give us more experience with the Dumas method along with also giving us more data points and possible a better percent error for other compounds. We could also experiment using other types of baths to help improve the system. For example, we could use a bath of highly salted water. In adding the salt to the water, the boiling point is increased, thus causing us to be able to evaporate liquids with a boiling point higher than that of water’s. However, this may actually prove slightly useless as high concentrations of salt are needed to significantly increase the boiling point. With these new experiments and our changes implemented, we would be able to turn an already fairly successful lab into an extremely successful lab.
Citations:
Chastain, B. B. (2013, June 18). Jean-Baptiste-André Dumas. Retrieved December 14, 2017, from https://www.britannica.com/biography/Jean-Baptiste-Andre-Dumas#ref157500
Overall, we were fairly successful in using the Dumas Method to determine the molar mass of the unknown volatile liquid. In trial 1, we found that from the sealing of the tube, 1.048 grams of condensed liquid formed and that the gas had taken up 271 mL of the flask from the volume that the water filled. The experiment was performed at a pressure of 761.5 torr and a boiling water temperature of 269.0 K. From this data, we were able to use the Ideal Gas Law to determine that the molar mass of the volatile liquid was 116 grams per mole. While this number is relatively close and isn’t an atrocious value, we still had a 61.9% error when our experimental molar mass was compared to the actual molar mass of 2-butanol (72.104 grams per mole). Though our first trial was semi-successful, our second trial was atrocious as we were not able to record data for the volume of the gas. This is because, in the process of sealing, the tip did not completely close leading to the pressures normalizing and the gas not being able to condense and form the partial vacuum needed to suck up the water. Because of this error in our second trial, we only had one data point to work with which limited the amount of analysis that we could have done.
Even in our first trial, there were many errors that could have affected the experiment. For example, when forming the special glassware tip, we may have made the curve too sharp so that the vapor of the volatile gas is unable to actually escape and push the air out. This error is easily fixable as we could have taken more time to make a perfect tube which would then give us more accurate results as the air would be more readily pushed out through the more slight curve than the sharper curve and there would be far less air in our partial vacuum. Another error was the fact that after taking the flask out of the boiling water and before sealing the tip, air may have leaked in through the tip, causing the partial vacuum to be even more partial and thus allowing less water to flood the flask, giving us an inaccurate value for the volume of the gas. While there is a relatively easy way of resolving this, it does come with a small amount of danger. In theory, this could be solved by sealing the tip while the flask is still in the water. However, the sudden pressure difference between the inside and outside of the flask may cause the flask to explode. Another way to resolve this would be to use a smaller bunsen burner that could be closer to the heating plate, thus lowering the time that the open tip of the flask is exposed to the air. A third error was the fact that we may have heated the volatile liquid for too short of a time. It was very hard to tell when the liquid had fully evaporated even when pulling the flask out or to even know if the flask had reached thermal equilibrium. If the liquid had not fully evaporated, it may be possible that not all the air in the flask was pushed out as there was not enough vapor to push it out. If the flask hadn’t reached thermal equilibrium with the water, the temperature value of the water would not be correct when plugged into the Ideal Gas Law and would yield an incorrect molar mass. This is easily fixable by allowing the flask to heat for longer and to more occasionally check the liquid level inside the flask to make sure it has all evaporated before removing it.
In addition to fixing the errors of our experiment, there are also other ways to improve it. For example, if we had more time, we could conduct more trials and be able to perfect our glassware. The extra trials and the better glassware would also yield more accurate results. In addition to this, we could also conduct trials with different compounds of other molar masses. This would give us more experience with the Dumas method along with also giving us more data points and possible a better percent error for other compounds. We could also experiment using other types of baths to help improve the system. For example, we could use a bath of highly salted water. In adding the salt to the water, the boiling point is increased, thus causing us to be able to evaporate liquids with a boiling point higher than that of water’s. However, this may actually prove slightly useless as high concentrations of salt are needed to significantly increase the boiling point. With these new experiments and our changes implemented, we would be able to turn an already fairly successful lab into an extremely successful lab.
Citations:
Chastain, B. B. (2013, June 18). Jean-Baptiste-André Dumas. Retrieved December 14, 2017, from https://www.britannica.com/biography/Jean-Baptiste-Andre-Dumas#ref157500
