THE NUMBER OF ATOMS IN THE SULFUR MOLECULE* HOWARD JAMES, 905 LINCOLN PLACE, BOULDER, COLORADO The molecular weight of sulfur is to a greater degree than that of any other element dependent upon the temperature. Below the boiling point there are eight atoms in the sulfur molecule. Somewhat above the boiling point the molecule becomes smaller. At a temperature of 467.goC. the vapor density of sulfur is 114.89 and a t 860" it is only 32.3. At 860°, therefore, the molecule contains 2 atoms, as the molecular weight is double the vapor density. Above this temperature, the vapor density remains constant until about 1800' when the molecule begins to dissociate. At 2000' and 380 mm. pressure the dissociation reaches about 45%. The determination of the density of sulfur vapor is too difficult for the average experimenter, but another method, applied to solid sulfur, can be carried out with little difficulty and fair accuracy. This method is based on the fact that one gram molecule (the molecular weight in terms of grams) of any substance dissolved in an equal amount of the same solvent lowers the freezing point of that solvent an equal number of degrees. It is evident, then, that by measuring the depression of the freezing point of a solution of known concentration the molecular weight of the solute can be calculated if the molecular lowering of the solvent is known. The molecular lowering is the number of degrees that one gram molecule of any substance dissolved in 1000 g. of solvent lowers its freezing point. The apparatus used for the determination is set up as follows. Select two test tubes of such size that one will fit within the other and hang suspended by its rim. The purpose of the outside tube is to produce a dead air space around the inside one and thus prevent sudden changes of temperature. Fit a two-hole stopper to the inside tube. Place an accurate thermometer through one hole and a small stirring rod through the other. The stirring rod should fit loosely enough that i t can easily be worked up or down, and its end should be bent to form a semicircle around the thermometer. To carry out the experiment, place 5 g. of pure naphthalene in the inner tube and suspend the whole apparatus in a beaker of water. The water level should be slightly higher than the level of the naphthalene when melted. Heat the beaker until the naphthalene melts, then remove the source of heat. Be sure that the whole bulb of the thermometer is immersed in the naphthalene and that i t does not touch the sides of the test tube in any place. Stir the molten chemical continually as i t cools. At a temperature of around 80' flakes of solid naphthalene will begin to form and the temperature will remain constant until the chemical has solidified. Determine this temperature to a tenth of a degree by m a n s of *Winner of five-dollar award in the student contest closing February 15, 1829.
VOL.6, No. 3
THE CHEM~STRS STUDENT
557
a good reading glass or other appliance. Remelt the naphthalene as above and dissolve in it exactly 0.5 g. of flowers of sulfur. Determine the point of solidification of this solution as above. To determine the molecular weight of sulfur from the data obtained, divide the number of degrees the freezing point was lowered by 100 (to find the number of degrees 1 g. sulfur in 1000 g. solvent would lower the freezing point) and divide the figure thus obtained into 7.1 (the molecular freezing point lowering for naphthalene). The results will only be approximate with the crude apparatus used, but if the experiment was carried out as accurately as possible, the figure obtained should fall between 254 and 263. These figures are approximately 8 times the atomic weight of sulfur, 32.064, thus showing that the molecule contains 8 atoms a t the temperature a t which naphthalene melts. The above method can be used for determining the molecular weight of any substance for which a suitable solvent can be found. The molecular lowering is, however, different for each solvent.
