QUANTITATIVE EXPERIMENTS IN ELEMENTARY CHEMISTRY. I. DETERMINATION OF THE MOLECULAR WEIGHT OF OXYGEN
The standard method of determining the molecular weight of oxygen by heating potassium chlorate i s criticized. A method utilizing sodium peroxide and water i s outlined. Comparative results of tests on both methods are giwen. The standard experiment of determining the molecular weight of oxygen in general chemistry laboratory courses invariably utilizes potassium chlorate as the source of oxygen. While the method in principle is good, there are certain inherent difficulties which are often the source of trouble to a freshman. In the first place the potassium chlorate should be dried, which is usually neglected or I . .. incompletely done. The experiment also requires a hard glass test tube, sufficientnumbers of which are not always available for large classes. The heating of the chlorate requires the most attention, because if impure or if suddenly heated to a high temperature, the salt may detonate. There is also occasional trouble with sublimation of the potassium chloride formed. With a view to shortening the experiment and obviating the above difficulties it was decided to use sodium peroxide as the source of oxygen. ' Details of the experiment follow. Assemble the apparatus shown in the accompanying diagram. The following will be required: a 5-cc. graduated pipet, a 7-inch test tube, a 500-cc. flask, a 400-cc. beaker, 1foot of rubber tubing, 2 feet of glass tubing for connections, 2 pinchclamps, and a thermometer. Draw water up into the pipet and then tighten pinchclamp to hold water a t upper limit of graduations. Place about 1.5 g. of sodium peroxide in the dry test tube, carefully transferring the material by means of a clean, dry, porcelain spoon. Stopper the tube with a cork and then weigh it accurately. The tube may readily be suspended on the balance arm by means of a wire ring and hook.
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1862
VOL.8, NO.9
ELEMENTARY CHEMISTRY.
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1863
When ready to start the experiment, fill the flask to the neck with water. Also fill the siphon tube with water, by sucking water into the tuhe and then closing the pinchclamp. Adjust conditions so that the air in the flask is under atmospheric pressure. This is done by placing a small beaker of water under the delivery tube, opening the pinchclamp and raising the beaker until the level of water in the flask is the same as that in the beaker. Now tighten the pinchclamp and place a dry 400-cc. beaker under the delivery tube. Connect the weighed test tuhe to the apparatus and see that all joints are tight. Open the pinchclamp controlling the delivery tuhe, and almost immediately begin the generation of oxygen by allowing water to drop slowly from the pipet. Control the addition of water to the test tube, so that no oxygen escapes by way of the pipet. Considerable pressure is developed and a volume of water equal to the oxygen liberated is forced from the flask into the beaker. After the action has slowed up add several more cc. of water from the pipet into the test tube. Record the volume of water added.* Allow the tube to cool t o room temperature (explain the increase in temperature in the tube) and then weigh as a t the beginning of the experiment. The first weight, plus the weight of water added, minus the last weight represents the weight of the oxygen evolved. Remove the beaker of water, after first adjusting the levels for atmospheric pressure in the flask. The volume of water may be obtained by weighing the beaker plus water on a platform scale, and then subtracting the weight of the dry beaker. If a large graduate is available, the volume can be more quickly measured directly. Record the temperature of the water and also obtain the atmospheric pressure from a barometer. Enter all data in your notebook and then calculate (1) the weight of a liter of oxygen and (2) the molecular weight of oxygen.
Data Weight of test tuhe plus sodium peroxide plus cork Weight of water added Weight of test tube plus cork (after reaction) (6) b - c) ( d ) Weight of oxygen evolved (a (e) Weight of beaker plus water Cfi Weight of beaker Volume of water = volume of oxygen (g) ( h ) Temperature (i) Barometric pressure ( j ) Aqueous tension at temperature of experiment (k) Volume of oxygen at 0°C. and 760 mm. Weight of 1 liter of oxygen at standard conditions (1) (rn) -Molecular weight of oxygen (a) (b)
+
=
= = =
* The weight of water added can be assumed very nearly equal to the volume of water dropped from the pipet. However, greater accuracy can be obtained by previously calibrating the weight-volume relation of the pipet.
1864
JOURNAL OF CHEMICAL EDUCATION
SEPTEMBER. 1931
The following data are submitted to show the relative merits of this method as compared to the chlorate method. Two groups of students were selected a t random from the elementary laboratory. One group of nine students was required to determine the weight of a liter of oxygen by heating potassium chlorate (the standard method). The results were as follows: 1.46, 1.32, 1.38, 1.18, 1.48, 1.43, 1.45, 1.54, and 1.31, showing an average value of 1.39. The second group of seven students, who really had less experience with quantitative experiments than the first group, was required to use the sodium peroxide method as outlined above. The values obtained for the weight of aliter of oxygen were: 1.44, 1.40, 1.43, 1.39, 1.43, 1.47, 1.44, the average being 1.43.
