Determining the Molar Mass of a Volatile Liquid by the Dumas Method
Texas A&M University Corpus Christi
General Chemistry II Laboratory
Mr. Arnold Mendez
February 11, 2010
Objective: The purpose of this lab was to determine the density of the vapor of an unknown volatile liquid. Use the calculated density and the ideal gas equation to calculate the molar mass of the liquid.
When a liquid is easily able to be converted to a gas, it is known to be a volatile substance. This sudden liquid to gas conversion is called vaporization, and thus, the gas formed is referred to as a vapor. However, these volatile liquids cannot be merely differentiated based on their appearance alone. Mainly because all of them look so similar to each other -clear and colorless. There is much more information needed to interpret the actualities of the volatile substance. One such piece of information is the molar mass of the substance.
In order to find the molar mass, with simplicity, one must use the Dumas method. In the Dumas method, we heat a sample of the liquid in a flask with a tiny opening until the entire sample vaporizes. Because the volume occupied by the vapor at atmospheric pressure is much larger than the volume occupied by the liquid, some of the vapor will escape from the flask. However, the vapor remaining in the flask will contain the number of moles of the substance that fills the volume of the flask at the experimental pressure and vapor temperature.
The ideal gas equation is expressed by the relationship among pressure (P), absolute temperature (T) in Kelvin, volume (V), and the number of moles (n). This equation is shown below:
PV = nRT
In the equation expressed above, R is a proportionality constant, in which the value depends on the units involved. When pressure is expressed in atmospheres and volume is in liters, R is 8.21 x [pic]L atm [pic] [pic]. J.B Dumas was the first person to recognize that we could use the...