Dumas Method
Chemistry 11
Santa Monica College
Determination of Molecular Mass via the Dumas Method
Objectives:
• To determine the molecular mass of an unknown volatile liquid using the Dumas method and the ideal gas law.
Background:
In the early 19th century, Jean-Baptiste Dumas, a distinguished French chemist, created a relatively simple method for determining the molecular mass of a volatile substance. In this experiment we will use a modified version of his technique to determine the molecular mass of an unknown volatile liquid. The density of a gas is given by the ideal-gas equation as,
Dgas =
m PM = V RT
where M is the molecular mass of the gas. Solving for molecular mass, we obtain:
€
M=
mRT PV
Thus, the molecular mass of a gas can be determined by measuring the temperature, pressure, mass, and the volume of a substance in its gaseous phase.
€
In this experiment we shall use a 125-mL Erlenmeyer flask in place of the glass bulb used by Dumas. A small sample of an unknown volatile liquid will be placed in the flask and the liquid vaporized by immersing the flask in a hot water bath. A piece of aluminum foil will be used to seal the flask and a tiny pinhole made in the foil to allow excess vapor to escape. The temperature of the gas will be determined by measuring the temperature of the water bath surrounding the flask and assuming that the gas and the water bath are in thermal equilibrium. Because an excess initial quantity of liquid is used, the volume of vapor produced from the liquid is greater than the volume of the flask. Upon heating, the vapor that is created initially pushes the air out of the flask and then the vapor begins exiting the flask through the pinhole until the pressure inside the flask is equal to the atmospheric pressure. Thus, the pressure of the gas can be determined by measuring the air pressure in the laboratory using a barometer. After the last of the liquid is vaporized the flask is removed from the hot
Santa Monica College
Determination of Molecular Mass via the Dumas Method
Objectives:
• To determine the molecular mass of an unknown volatile liquid using the Dumas method and the ideal gas law.
Background:
In the early 19th century, Jean-Baptiste Dumas, a distinguished French chemist, created a relatively simple method for determining the molecular mass of a volatile substance. In this experiment we will use a modified version of his technique to determine the molecular mass of an unknown volatile liquid. The density of a gas is given by the ideal-gas equation as,
Dgas =
m PM = V RT
where M is the molecular mass of the gas. Solving for molecular mass, we obtain:
€
M=
mRT PV
Thus, the molecular mass of a gas can be determined by measuring the temperature, pressure, mass, and the volume of a substance in its gaseous phase.
€
In this experiment we shall use a 125-mL Erlenmeyer flask in place of the glass bulb used by Dumas. A small sample of an unknown volatile liquid will be placed in the flask and the liquid vaporized by immersing the flask in a hot water bath. A piece of aluminum foil will be used to seal the flask and a tiny pinhole made in the foil to allow excess vapor to escape. The temperature of the gas will be determined by measuring the temperature of the water bath surrounding the flask and assuming that the gas and the water bath are in thermal equilibrium. Because an excess initial quantity of liquid is used, the volume of vapor produced from the liquid is greater than the volume of the flask. Upon heating, the vapor that is created initially pushes the air out of the flask and then the vapor begins exiting the flask through the pinhole until the pressure inside the flask is equal to the atmospheric pressure. Thus, the pressure of the gas can be determined by measuring the air pressure in the laboratory using a barometer. After the last of the liquid is vaporized the flask is removed from the hot