The Dumas method of molecular weight determination was historically a procedure used to determine the molecular weight of an unknown substance. The Dumas method is appropriate to determine the molecular weights of volatile organic substances that are liquids at room temperature.
The method was designed by a French chemist named Jean Baptiste André Dumas, after whom the procedure is now named. Dumas used the method to determine the vapour densities of elements (mercury, phosphorus, sulfur) and inorganic compounds.
The procedure entailed placing a small quantity of the unknown substance into a tared vessel of known volume. The vessel is then heated in a boiling water bath; all the air within the flask would be expelled, replaced by the vapor of the unknown substance. When no remaining liquid can be observed, the vessel may be sealed (e.g. with a flame), dried, and weighed.
By subtracting the tare of the vessel, the actual mass of the unknown vapor within the vessel can be calculated. Assuming the unknown compound obeyed the ideal gas equation, the number of moles of the unknown compound, n, can be determined by
By dividing the mass in grams of the vapor within the vessel by the calculated number of moles, the molecular weight may be obtained.
Two major assumptions are used in this method: