Mahesh Godavarti

BTW, this method is named after Alexander Dumas and it is commonly referred to as Dumas Method. If you want to use the possessive "'s", then it should be referred to as Dumas' Method or Dumas's Method and NOT Duma's Method. Duma's implies the last name was Duma which would be incorrect.

Dumas's Method is used to calculate the mass of each molecule (molecular weight) of a substance. It creates a situation where we can guess at the number of molecules of the substance and by weighing that substance we can estimate the molecular weight of the substance (by simply dividing the measured mass by the estimated number of molecules).

So, how is this done?

You first turn the substance into gas/vapor (by heating it). Gas/vapor has mostly predictable behavior and approximately follows the ideal gas law of PV = nRT where P is the pressure the gas exerts, V is the volume, n is the number of moles (used to calculate the number of molecules), R is a constant and T is the temperature.

The substance is first placed in a vessel of known volume and mass and heated till it vaporizes completely. At this point, it is assumed that all air in the vessel has been replaced by the substance's vapor. At this point, the vessel is sealed and the combined mass of the gas/vapor and the vessel is measured. By subtracting the mass of the empty vessel from the combined mass you get the mass of the gas/vapor. At the same time you also measure the pressure exerted and the temperature of the gas and use the ideal gas law to calculate the total number of molecules in that volume.

Now that you have both the total mass and the total number of molecules of the substance, you can find the molecular weight of the substance.

BTW, this gives rise to other interesting questions as to how the exerted pressure and the mass of the empty vessel are measured. If you have that question, feel free to post that as well.