Dumas Method
Introduction
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
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
References: Cerpovicz, P. and Wojciechowski, B. (1998) “Intermolecular Forces” 28 October 2007. Giunta, C. (2003) “Dumas Vapor Density” 28 October 2007. < http://web.lemoyne.edu/~GIUNTA/classicalcs/dumas.html> McGuigan, B. (2007) “Wisegeek” 28 October 2007.