JOURNAL oa CHEMICAL EDUCATION
152
SEPTEMBER, 1924
THE VAPOR DENSITY OF STEAM* W. L. EVANS, J. E. DAY,C. S. PEASE AND G. D. BLAND, Orno STATE UNIVERSITY, COLUMBUS,OHIO
The classical experiments of Berzelius and Dumas by means of which these investigators showed that the elements hydrogen and oxygen unite to form water in the ratio of 1 : 7.98, respectively, do not furnish sufficient data to enable the beginner in chemistry to determine the exact formula for water. In addition to this quantitative data with reference to the composition of water, it is necessary that the vapor density of the compound be known in order that its molecular weight may be calculated. Obviously it is quite impractical for the beginner to actually weigh one liter of steam. However, we have found that this difficultyis overcome by transferring a known volume of steam into a small calcium chloride tube which has been previously weighed. The following are the details of the experiment.
P
11
Two lifer beoker
250
CG.
I
CorJcd HJO,.
rubber:
Fig. 1. An apparatus is arranged in accordance with Pig. 1. Before the heating of the brine (CaC12)is begun, the U-tube is disconnected a t B and C, and the soda lime-calcium chloride tube a t A, the precaution being taken to close these tubes with the well known plugs made of glass rodding and rubber tubing. The U-tube is now weighed and then attached to the straight calcium chloride tube a t C. The screw clamp should be opened. When the temperature of the brine * Read before the Division of Chemical Education of the American Chemical Society at the M'iaukee Meeting, Sept. 13, 1923.
is 106-110°, add 3 or 4 cc. of hot water to the flask. After the water has evaporated, this operation should be repeated a t least twice in order to insure the complete removal of the air. Continue the heating until there is no water in the flask or tubes. Immediately attach the straight drying tube a t A and the U-tube a t B and then close the screw clamp tightly. Start the suction pump and regulate i t in such a manner that 2 to 3 bubbles of air per second pass through the sulfuric acid drying bottle. By this means dry carbon dioxide free air will be drawn through the apparatus and the steam will be swept into the U-tube. The temperature of the bath should be kept up during this operation. When the steam has been completely carried into the U-tube (apply suction for about 40 minutes), disconnect this tube, insert the glass plug and reweigh. The gain in the weight of the U-tube is the weight of the known volume of steam. The following gram molecular weights of steam were calculated from experimental data obtained by some of our students: 19.02; 18.45; 15.9; 21.0; 18.02; 17.36; 17.96; 19.36; 18.0; 17.90. Knowing the percentage composition of water and its gram molecular weight the student could easily show that the formula of water is correctly represented by HzO.
