To determine the molecular weight of a volatile liquid
To determine the molecular weight of a volatile liquid, we used a method that known as the ‘Dumas Method’. Jean-Baptiste Andre’ Dumas, in 1826, devised a method, likewise dependent on the Hypothesis of Avogadro, for determining the Molecular Weights of fluid substances that can be conveniently be turned into vapours. The Dumas Method was one of the first techniques available for the measurement of the Molecular Weights of compounds. This was a major step towards being able to determine the Chemical Formulas of these compounds. (The Molecular Weight)
To apply the ‘Dumas Method’, firstly, we weighed a clean and dry 125 Erlenmeyer flask together with a piece of an aluminium foil and a rubber band which is 54.012 g. Then, 3 mL of 2-propanol is placed in the Erlenmeyer flask with a drop of iodine to coloured the 2-propanol fluid. The Erlenmeyer flask is covered by an aluminium foil and placed a rubber band around it. Next, a pin-sized hole pierced in the aluminium foil to give a movement of air and gas from the Erlenmeyer flask. After that, the flask who is containing the 2-propanol is placed into the boiling water bath with a utility clamp. The water level is adjusted high on the neck of the flask so that it covered as much as possible of the Erlenmeyer flask.
When the flask is placed, the fluid vaporized and pushed all the air out of the flask. Vaporized gas will escape from the jar until such time as the pressure of the gas in the flask equals the pressure of the atmosphere. Then, removed the flask after measured the temperature with the thermometer and placed it on a clean dry surface to cool to the room temperature before weighed it with a small amount of fluid that condensed inside the flask. As the result, we could measure the volume and mass of an unpredictable fluid at the boiling temperature of water and ambient pressure after weighed the flask and its content which is 54.268 g. this experiment is repeated two more times.
For the average molecular weight of 2-propanol that we obtained is 62.65 g/mol. Theoretically, we supposed to get 60.1 g/mol of the molar mass of the 2-propanol so this shows that there might be a few mistakes that have been done during the experiment. One of the possible error is the amount of heating time that was applied to the sample of volatile liquid. Though heating time was made as short as possible and was only until the volatile liquid was observed to be all vapour, the time may have been too long. During the lab, it was noted that the little bit of volatile liquid in the Erlenmeyer flask was rather hard to see when it was submerged in the hot water bath, despite the tilting of the flask. This made it hard to say so for sure when all of the liquid was vaporised. When the flask was removed, all of the entity was observed to be vapour. However, it is very possible that because there was great difficulty in observing the liquid in the flask, there was too much heating time given. Because of the pinhole in the aluminium foil, excess boiling time would have caused a larger quantity of the vapour to escape than if the liquid was only heated as much as necessary. This would have carried on to being an error when the mass of the flask containing the condensed fluid was taken, as less entity than before would directly translate to less mass. This would have also created error in the molar mass calculation, as mass and molar mass are directly proportional.
Another error that might happened is that the hot water bath temperature was not simple to control, and occasionally either went up or down by approximately 1.0ºC. Despite the fact that generally the temperature was steady and noted to be 80.0ºC, there were changes that must be made a couple of times all through the methodology to keep the temperature stable. If however, the temperature was not averaged to be 80.0ºC like the recorded value, the recorded value would have created error in the calculation for determining the number of moles (using the ideal gas law). From there, the error would have conveyed forward and created error in the later calculations dependent on the ideal gas law, such as the molar mass calculation, the volume of vapour at STP count, and the sub-atomic mass calculation.