Dumas Method

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  • Topic: Gas, Molecular mass, Pressure
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  • Published : October 13, 2008
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In order to identify new materials, scientists use a variety of chemical and physical methods to determine molecular masses. One of these methods includes the Dumas method for determining the molecular weight of a volatile liquid. This method, which was proposed by John Dumas in 1826, makes use of a volatile liquid (vaporizes at a relatively low temperature) and allows this liquid to be heated in a water bath to a known temperature and escape from a flask through a tiny opening (Giunta, 2003). In this situation, vapours are assumed to be obeying the Ideal Gas Law, which is PV = nRT. P is the current atmospheric temperature, measured in atmospheres, V is the volume of the flask, n is the moles of the gas and T is the temperature of the water bath, measured in degrees Kelvin (Weisstein, 2007). R remains a constant, which is 0.08206 L atm/K mol (Weisstein, 2007). The equation used to solve for n (mass/molecular weight) is substituted into the Ideal Gas Law and then rearranged in order to solve for molecular weight (MW). As a result, the equation used in the Dumas Method is as follows: MW = mRT/PV (Giunta, 2003). The Dumas method depends on a lot of things to go right. For example, the liquid used in this experiment must be volatile enough to vaporize at high temperatures (Giunta, 2003) On the other hand, the liquid must not be too volatile to ensure that a significant amount of the liquid to evaporate as the flask cools (Giunta, 2003) Therefore, liquids with weaker intermolecular forces, which are forces that hold molecules together (Cerpovicz and Wojciechowski, 1998) would have less error than those liquids containing stronger intermolecular forces. For this experiment, two volatile liquids were used. The first was methanol and the second was an unknown liquid, which may have been methanol, ethanol, iso-propanol and hexane. Methanol is a clear, flammable and toxic liquid which is sometimes used as a vehicle fuel. Its accepted molecular weight value is 32.04 g/mol (McGuigan, 2007). Ethanol is a clear, colourless liquid that is miscible with water and other organic compounds. Its accepted molecular weight value is 46.06 g/mol (McGuigan, 2007). Like ethanol, iso-propanol is clear colourless liquid that is miscible in water. It is also flammable and very toxic. Its accepted molecular weight value is 60.10 g/mol (McGuigan, 2007). Finally, hexane, like the other volatile liquids, is a clear, flammable liquid and its accepted molecular weight value is 86.18 g/mol (McGuigan, 2007). All of these liquids must be handled very carefully because of their flammability characteristics. The purpose of this experiment was to determine the molecular weight of the methanol solution and the molecular weight of an unknown solution, after which to identify the specific volatile liquid using this value. With the calculated molecular weights, a percent error can be determined by comparing the molecular weights calculated and the accepted molecular weights of the four volatile liquids. A percent error must have a positive number, so the subtraction part of the equation can be reversed (absolute value of the difference) (Curran, 2007). Percent error is determined using the following equation: | (Calculated Value-Accepted Value) |/ (Accepted Value) *100 (Curran, 2007)

The experimental procedure used for this experiment was outlined in the CHEM 120L lab manual, under Experiment #2. All steps were followed with no changes in the procedure.

Results and Calculations
Table 1: Observations for Determining the Molecular Weight of Methanol
Original Mass
of Flask
(g)Mass of Cool
Assembly After Heating
(g)Flask Volume
(mL) Water Bath
Trial 185.365685.4231155.599.911.23
Trial 293.878994.070115898.836.64
Trial 397.988098.100015398.522.15...
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