determining mass of a voltaile liquid
Discussion
In this experiment, the molar mass of two volatile liquids – methanol and an unknown liquid – were calculated for through the Dumas Method.
Initially, the sets of Erlenmeyer flasks with aluminum foil and rubber bands were weighed. Next, volatile liquids were placed in 125mL Erlenmeyer flasks and covered with aluminum foil, but small holes were made into the foil. The flask was then heated in a water bath until all of the liquid evaporated. The flask was then removed, and weighed again after being cooled to room temperature.
To determine the molecular weight of the liquids, the mass of the condensed vapour is needed. This is achieved through the heating and cooling down of the flask – because the holes made in the foil allowed the excess vapour to escape, the methanol/unknown vapour pushed out the extra vapour and oxygen gas. When the flask was cooled down, the methanol/unknown vapour condensed – this resulted in the increase in mass of the flask when it was weighed after being cooled down. The mass of the condensed vapour is equal to the difference between the initial and final masses of the flask. Due to the assumption that the volatile liquids obey the Ideal Gas Law: PV=nRT, where n is equal to the mass of the substance divided by the molar mass – giving M=mRT/PV, the molecular weight could then be calculated for.
The experimental results revealed an average molar mass of 31.10g/mol for methanol (theoretical value of 32.04g/mol) and an average molar mass of 60.28g/mol for unknown # 20. which is closest to the molar mass of iso-propanol, 60.10g/mol. The differences in the molar masses of the experimental values and theoretical values may have been to sources of error such, as weighing the flask before it has fully cooled down to room temperature, or allowing too much vapour to escape from the flask during heating. Each of these factors would result in a higher or lower molecular weight than the theoretical value.
In this experiment, the molar mass of two volatile liquids – methanol and an unknown liquid – were calculated for through the Dumas Method.
Initially, the sets of Erlenmeyer flasks with aluminum foil and rubber bands were weighed. Next, volatile liquids were placed in 125mL Erlenmeyer flasks and covered with aluminum foil, but small holes were made into the foil. The flask was then heated in a water bath until all of the liquid evaporated. The flask was then removed, and weighed again after being cooled to room temperature.
To determine the molecular weight of the liquids, the mass of the condensed vapour is needed. This is achieved through the heating and cooling down of the flask – because the holes made in the foil allowed the excess vapour to escape, the methanol/unknown vapour pushed out the extra vapour and oxygen gas. When the flask was cooled down, the methanol/unknown vapour condensed – this resulted in the increase in mass of the flask when it was weighed after being cooled down. The mass of the condensed vapour is equal to the difference between the initial and final masses of the flask. Due to the assumption that the volatile liquids obey the Ideal Gas Law: PV=nRT, where n is equal to the mass of the substance divided by the molar mass – giving M=mRT/PV, the molecular weight could then be calculated for.
The experimental results revealed an average molar mass of 31.10g/mol for methanol (theoretical value of 32.04g/mol) and an average molar mass of 60.28g/mol for unknown # 20. which is closest to the molar mass of iso-propanol, 60.10g/mol. The differences in the molar masses of the experimental values and theoretical values may have been to sources of error such, as weighing the flask before it has fully cooled down to room temperature, or allowing too much vapour to escape from the flask during heating. Each of these factors would result in a higher or lower molecular weight than the theoretical value.