<P> Today, modern methods such as mass spectrometry and elemental analysis are used to determine the molecular weight of a substance . </P> <P> 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 . </P> <P> 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 </P> <Dl> <Dd> p V = n R T (\ displaystyle pV = nRT \,) </Dd> </Dl>

How molecular weight of organic compound is determined by platinichloride method