The molar mass of a test fluid acetone was determined via the measurement of the vapor density at a mean water boiling temperature of 98.3°C and a mean pressure of 734.6 torr. The molar mass of acetone was found to be 73.13 ± 22.3 g/mol compared to the known value of 58.08 g/mol, hence, yielding a percent error of 25.90 %. Major source of error is due to the apparatus not being completely dry within every trial in order to yield the most effective results.
BACKGROUND: The mass of a substance expressed in grams in terms of moles is known as the molar mass1. There are various methods in which the molar mass can be determined. First method is the vapor density method, a known value is weighted and the volume is converted to STP, hence molar mass can be determined. Another method is the cryoscopic method in which a known quantity of a substance is added to a solvent, hence the freezing point is measured, specific calculations will result in the molar mass. Vapor pressure osmometry is another method; a known amount of a substance is dissolved into a solvent, therefore the difference in vapor pressure is recorded2.
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PROCEDURE: Hirko, R. Chemistry 112L General Chemistry I Laboratory, 6th ed.; bluedoor: Eden Prairie, MN, 2012; Experiment 6.
Table 1 shows three trials of vapor density reaction containing acetone. For each trial, the mass of the flask, foil and rubber was first recorded. A pressure probe and a temperature probe were used to obtain the pressure and temperature respectively. 2 mL of Acetone was added to a flask and the flask was placed inside the beaker containing the boiling water. Acetone was evaporated, the flask was let to cool done, then the flask containing the residual acetone was measured. The difference between both masses was recorded. The volume of the flask is equivalent to the
Table 1. Acetone
Mass flask, foil, and rubber band/g
Barometric pressure /torr
Water boiling T/°C
Mass flask, foil, and rubber band/g + residual test fluid/g
Residual test fluid/g
Volume of the flask/ml
Molar mass /(g/mol)
Mean molar mass/(g/mol)
volume of the gas, because the gas fills out the shape of the container. The molar mass was found by the following equation: M=mRT/PV, m is the mass of residual acetone, R is the gas constant, T is the temperature constant, P is the pressure constant and V is volume of the gas. The molar mass was found for the three trials as shown in table 1. Therefore, the mean molar mass was found to be 73.13 ± 22.3 g/mol compared to the known value of 58.08 g/mol, hence, yielding a percent error of 25.90 %.
DISCUSSION: Molar mass was found to be 73.13 ± 22.3 g/mol compared to the known value of 58.08 g/mol, the observed mass was found to be more than the known value, yielding a percent error of 25%, however, the observed mass still falls within the norm, because the uncertainty is ± 22.3 g/mol.
The molar mass results obtained within the three trials are 74.07 g/mol, 84.80 g/mol and 60.53 g/mol respectively. These observed values are inconsistent, that is attributed to the use of dropper to place the acetic acid in the flask, which varies within the three trials, i.e. despite the volume used is 2ml for all three trials, however, it is extremely difficult to obtain the exact same amount of concentration for each, therefore, the results are inconsistent. On the other hand, the percent of uncertainty compared to the determined molar mass is 30.5%. This percentage is within expectations because it makes the observed mean molar mass value fall within the norm of the known value.
Regarding the pressure and temperature observed; there is a directly proportional correlation between them, as the pressure increase the boiling point increases. During the trials, the pressure stayed relatively constant, with a tiny decrease/increase within each as shown in table 1, resulting in a mean of 734.6. For that reason, the temperature didn't fluctuate throughout the three trials, as a result the boiling point was fairly constant also. This is attributed to the weather in Brookings, if the pressure was high the values obtained would be altered, therefore, even if it is indirect, there is a correlation between the weather and the boiling point. For instance, if the same experiment was undergone in Santa Fe, New Mexico at an elevation of 6344 ft, the boiling point will decrease, hence the molar mass will also decrease. This is because at higher heights the pressure decrease, and as the pressure decrease the temperature of the water bath will also decrease, therefore the boiling point will be lower yielding to a lower molar mass.
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If the solvent was more polar the known value will increase depending on the increase of polarity, in other words, the more polar the solvent the higher the known value. This is because more polar substances have a higher boiling point due to dipole-dipole interactions between their molecules; hence deviation from the known occurs.
Major sources of error are attributed to the apparatus not being completely dry within every trial; this would alter the mass of the flask measured, as well as the beaker used etc. This would cause lack of consistency within the obtained results. The whole made in the middle of the foil was not small and tight enough, this will account for a faster rate of evaporation, hence the mass of residual acetone will increase, which means that the molar mass will be higher.
Potential improvements can be attributed to the use of an enhanced temperature and pressure probes, which would account for more reliable results, in addition, if an apparatus is to be used again it should be completely dried out, to ensure that accurate results are achieved.