The apparatus for the determination of the molecular weight of a dissolved substance from the elevation of the boiling point of a solvent, devised by Beckmann, is shown in.

Fig: Beckmanns apparatus Beckmanns apparatus for determination of elevation of boiling point.

The thermometer and the tube for holding the solution are the same as those used in the freezing-point apparatus except that the tube has a short piece of platinum wire sealed through the bottom to assist in the transmission of heat, and contains a layer of small crystals of garnet to prevent bumping. The tube is surrounded by a glass mantle plugged with asbestos, and the closed end of the tube with the platinum wire projecting is heated over a slightly smaller hole in a piece of asbestos millboard, with wire-gauze beneath, by means of a small Bunsen flame so as to get a uniform temperature. The solvent vapour is condensed in a reflux condenser formed of a limb of the tube, as shown, and the liquid flows back. The boiling point of a weighed amount of the pure solvent is first found on the Beckmann thermometer the mercury column having been suitably adjusted. The tube is then cooled, a weighed quantity of the substance the molecular weight of which is to be found is introduced through the side tube and completely dissolved, and the boiling point of the solution is found. The difference is D, the elevation of boiling point.

A more convenient apparatus is that of Landsberger, modified by Beckmann (1902), in which the solution is heated by passing through it the vapour of the solvent. The vapour condenses, giving out heat and raises the temperature of the solution until the boiling point is reached. The vapour then passes through without condensation. The glass tube A

Fig: Landsberger-Beckmann boiling-point apparatus

contains the solvent, and is heated in the same way as in the preceding apparatus. Inside is the tube B, graduated in c.c., containing the solvent and the thermometer. Vapour from A bubbles through the solvent by way of the tube C, open to A. The tube R prevents liquid from B being sucked back into A. The vapour is condensed in E, and the liquid can be allowed to flow back into B, or by turning the condenser in the ground joint, returned to A through the siphon-tube F, the opening of which is brought opposite a hole shown. When the solvent in B boils the temperature is read off. The flame is then removed, a weighed quantity of the substance introduced through the side arm, and the heating repeated until the solution boils. The temperature and volume of the boiling solution are read off. The molecular weight is calculated from the formula:

where w= weight of solute, v = vol. of solution in c.c., E = observed elevation of boiling point, K = a constant. The values of K for some common solvents are: water, 540; alcohol, 1560; acetone, 2200; ether, 3030; benzene, 3280