APPROXIMATE MOLECULAR WEIGHTS FROM THE BOILINGPOINT RISE: A LABORATORY EXPERIMENT IN ELEMENTARY CHEMISTRY* ~ T H U R R
o s ~ AND t R. D. BILLINGER, LEAIGH UNIVERSITY, BETHLE~M, PENNSYLVANIA
This experiment was designed to give students first-hand experience with such topics as the effect of a solute on the boiling point of a liquid and the calculation of approximate molecular weights. The problem could be presented to the students in the following form: This compound has been found on analysis to contain z% C and y% H. Calculate the empirical formula and by means of boiling-point determinations find the approximate molecular weight of the compound and write the molecular formula. I t is well to emphasize at the start that the method gives only approximate molecular weights and is used to determine which multiple of the formula weight is the exact molecular weight. In a general way this experiment is similar to and illustrates the same principles as freezing-point experiments. However the latter have certain disadvantages when attempted in an elementary laboratory. On an ordinary thermometer the zero mark is always near the bulb. As a result it is difficult to get accurate readings in a freezing-point experiment since the bulb and lower part of the scale are inside the apparatus and often under the solution. Supercooling effects are impossible to prevent and these confuse the elementary student. The use of-stirrers is essential and this complicates the apparatus. Finally the melting i y and salt mixtures are invariably spilled over the desk and result in a messy laboratory. All these disadvantages are overcome by use of the following boilingpoint experiment. The apparatus and procedure are so simple that no special supplies are necessary and yet the experiment is a test of the student's ability to set up apparatus and get results in the laboratory. The character of the experiment and the precautions to be observed are best indicated by stating the directions actually given to a class in general chemistry. Determination of the Boiling Point of Pure Carbon Tetrachloride Each student will need the following apparatus: Bunsen burner, ring stand, iron ring, wire gauze, clamp, 400 cc. beaker, clean dry 7-inch test tube with two hole cork to fit, &foot length of glass tubing, 2 ordinary (tllO°C. range thermometers and 20 two-inch lengths of capillary tubing (1 mm.inside diameter) sealed a t one end only. In order to determine the
* Essentially this same paper was presented before the Divi;ion of Chemical Education of the A. C. S. at the Atlanta meeting, April, 1930. t Present address is Pennsylvania State College, State College, Penna. 2715
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boiling point of pure carbon tetrachloride, set up the apparatus as shown in the diagram.' First put the clean dry capillary tubes2 (open end down) in the dry test tube and rinse with two portions of pure carbon tetrachloride. Then put the beaker in place on the ring stand and clamp the test tube in place so that its bottom is one inch from the bottom of the beaker. Next fit the cork with the thermometer and the glass tubing, which is to act as a condenser tube. The bulb of the thermometer should be one inch from the bottom of the test tube when the stopper is put in place. When all is adjusted pour 12 cc. of pure carbon tetrachloride into the test tube, put stopper in place, and nearly fill the beaker with tap water. Finally put the second thermometer in place so that it will record the temperature of the water. Heat the beaker with full Bunsen flame until the water reaches 80-85°C. Then remove the flame while the temperature gradually rises. During the remainder of the determination use a small Bunsen flame whenever the water temperature drops below 88", but stop heating as soon as the temperature goes above this. In this way the temperature of the water is 0'. kept between 85' and 9 As soon as the temperature of the water has been adjusted, begin to take readings on the thermometer in the test tube. Read every minute to tenths of a degree and record all the readings and corresponding times. Continue to take readings until the temperature remains practically constant for a five-minute period (variation should be less than three-tenths of a degree). Average the five values and record this temperature as the boiling point of the carbon tetrachloride. Approximate Molecular Weight of Naphthalene from Boiling-Point Rise Prepare a naphthalene solution by dissolving about 5 g. of naphthalene (weighed to tenths of a gram) in 25 cc. of carbon tetrachloride accurately It was found helpful to exhibit a model apparatus in the laboratory so that all students could see just what was necessary. Each student's apparatus was then inspected, approved, and graded before he actually began the determination. Capillary tubes were found much superior to glass beads in preventing superheating and variation in boiling point.
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measured from a graduated cylinder. Dismantle the test tube part of the boiling-point apparatus and pour out the carbon tetrachloride. Rinse the capillary tubes and test tube with two 5-cc. portions of the naphthalene solution just prepared. Then pour 12 cc. of the solution into the test tube and re-assemble the apparatus. Determine the boiling point of this solution exactly as was done for pure carbon tet~achloride.~ The difference in the boiling points of carbon tetrachloride and of the solution is the boiling-point rise for the solution. From this value and the molar-boiling-point elevation for carbon tetrachloride calculate the molarity of the naphthalene solution and the approximate molecular weight of naphthalene. Discussion The same procedure can be used for other boiling-point determinations, in each case keeping the heating liquid4 about ten degrees above the temperature of the boiling solution in the test tube. It is necessary to use rather concentrated solutions in order to get sufficient elevation for reasonably accurate observations with an ordinary thermometer. The solution mentioned in the above directions is about molar. It is easy to think of extending the method of this experiment to solutions of ionogens to illustrate and measure ionization. However, water is an unsatisfactory solvent because of its unusually small boiling-point constant. That it is such a common solvent aqdthe only available ionizing solvent should probably outweigh the above disadvantages but i t is difficult to get results showing an average error of less than 16 to 15% without introduction of special apparatus. Carbon tetrachloride has a considerable advantage over water and other solvents because it has an unusually large boiling-point constant (water 0.52, benzene 2.56, carbon tetrachloride 4.85) and because of its non-inflammability and low boiling point. Using the naphthalene-carbon tetrachloride combination, the better students can be expected to get results within two or three units of the correct molecular weight (CloHs = 128), while the average error for the class should be less than 5%. Since the experiment involves some concentra"he following alternative procedure can be used in preparing the naphthalene solution: Remove the test tube from its clamp and roo1 under the tap. When the carbon tetrachloride has cooled, remove the cork, and add 2 g. of naphthalene (weighed accurately t o tenths of a gram) to the carbon tetrachloride Then re-assemble the apparatus and determine the bailing point of the solution. This method avoids the error due to evaporation of solvent during rinsing of the test tube, but the error in measuring the smaller amounts of carbon tetrachloride and naphthalene becomes relatively more important. 4 For solutions having a boiling point greater than 100DC.,a bath of crude glycerin is probably the most satisfactory heatine liquid for general use in elementary laboratory work.
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tion and close observation, a few students will get very poor results or even fail to finish in the time allotted. For instance, in a section of seventeen students one student did not finish, another obtained a value of 160, while seven students had values between 123 and 133. The average error for the section was 3.1%. Although neither naphthalene nor carbon tetrachloride are familiar substances to the ordinary elementary student, a mention of their connection with moth balls and dry cleaning, respectively, should establish helpful associations in connection .with the experiment.
Rules for Reading. How and why should books he read? It is t o be feared that far too many read books merely t o kill time, or else for frothy entertainment. Even informative books are often hastily skimmed over to pick out that which interests, rather than t o absorb and retain the information which the author seeks t o impart. According t o Dr. Rallo L. Lyman of the University of Chicago, the following rules should be ohsenred in order to get the largest amount of good from reading. Dr. Lyman states that a carefnl reader: 1. Reads with a definite purpose, a problem, in mind. 2. Grasps the author's point of view and central theme. 3. Lays hold of the order and arrangement of the author's ideas. 4. Pauses occasionally for summarizing and repeating. 5. Constantlv asks auestions of his readina. . 6. Continually supplements from.his own mental stock. 7. Evaluates the worth of what he reads. 8. Varies the rate of his progress through the reading. 9. Ties up what he reads with problems of his own.-Kalcnds of the Waverly hss
Removing Metal Scale. After years of experimentation, a firm in New England has perfected a method of removal of scale resulting from heat-treatingaud hardening operations on metals, not only from Aat surfaces but from such intricate machine parts as gears, shafts, etc., as well as from cutting tools, such as drills, taps, dies, cutters, and reamers. The scale is removed without in any way injuring the surface structure of the steel or metal part. The nrocess is an electroehemica1one, the descaline - action beine - broueht . about by the generation of hydrogen on the work. a t the same time protecting the work from hydrogen embrittlement or pitting by coating the cleaned work by depositing a metal on it. This coating remains on the work, if i t will not interfere with subsequent operations. It can easilv be removed electrolvtically if found t o be undesirable. This process makes i t possible t o doawaywith the hand work formerly necessary, like filina, sand-hlastins, scrtipinz, and scratch hrnshiuz, and thus zreatly reduces the labor cost. Further studs is hcing made 01 the adaptation of this procvss to the special fields in the metal industry. It is expected that it mill have wry eide application The process is said t o be so simple that only a small installation is required, embracing a motor generator far the current and tanks in which the treatment is given.Ind. Bull., Arthur D.Litllc, Inc. ~
