SIMPLE APPARATUS for DETERMINING PERCENTAGE COMPOSITION LEROY D. JOHNSON Storer College, Harper's Ferry, West Virginia
N
UMEROUS experimenters have from time to time submitted modifications of the Dumas apparatus for determining the percentage composition, but
the following modification for the general college student has several unique features. It not only serves as an apparatus for determining the
percentage composition of water, but may be used equally well for determining the chemical equivalent of both oxygen and hydrogen, or may be used to determine the formula of copper oxide. In the second place this method partially reduces the error due to loss of water about stoppers which usually brings about low results in the experiments of this type. Thirdly, the cost of materials and apparatus as well as the possibilities of breakage of the latter must be reduced to a minimum in the small laboratory. There is a decided advantage in the method presented because the distilling flask and other pieces of apparatus may be used and reused with little possibility of breakage due to overheating or carelessness. Then, too, time plays an important part in the setting up of laboratory pieces of apparatus. It bas been found
that this apparatus can be assembled and the experiment completed in the usual laboratory period of two or three hours. Finally, since in the assembly of the apparatus and in the calculations involved there is a fair degree of accuracy obtainable themethodserves as ameans of partially estimating the ability of the student to become quantitatively minded. The procedure (see figure) and the results of actual experiments conducted are included to indicate the possibilities of the method. Hydrogen is generated in the usual manner by placing commercial zinc in a gas generating bottle fitted with thistle tube and outlet tube. The outlet tube is connected to a calcium chloride drying tube which in turn is connected by a piece of glass tubing to a 25- or 50-cc. Pyrex distilling flask. The glass tubing connecting the drying tube and the distilling flask extends to the back of the distilling flask. To the outlet arm of the distilling flask is attached a straight or U-shaped calcium chloride tube which has been carefullv weiehed to 0.01 a. This calcium chlo-
stopper a t one end and a small piece of rubber tubing with a screw clamp a t the other end. Into the distilling flask is placed about two or three g. of black copper oxide wire or powder. The flask is weighed with the copper oxide before the experiment begins. An ordinary laboratory burner of the Bunsen type serves to heat the copper oxide in the bulb of the distilling flask. After the hydrogen has shown to be free from air and a slow, steady generation of the gas results the clamp on the calcium chloride absorption tube (No. 2 in diagram) is opened, and the stopper in the other end of the tube is removed. The flask containing the copper oxide is then heated slowly. To avoid any condensation about the rubber stopper connecting the distilling flask and the drying tube i t is advisable to heat a t intervals the section labeled x-y in the figure or to place a small burner flame under this section. Care must be taken not to burn the stopper. As the reduction proceeds water will form and pass by gravitational force down the outlet tube of the distilling flask into the absorbent tube. When the reduction is complete the flame may be so directed to drive all remaining traces of moisture in the arm of the distilling flask into the absorption tube. The apparatus should not be immediately disconnected, but the copper oxide bulb should be allowed to cool down to room temperature with a stream of hydrogen passing through. After the tube has cooled sufficiently and all traces of water been driven into the absorption tube the generator may be replaced by an empty generator bottle which is attached to the calcium chloride drying tube. Into this empty generator bottle, fitted with thistle tube and outlet tube, is poured some water to replace the hydrogen in the tubes with air. After this is done the absorption tube is immediately closed with a stopper in one end and the clamp on the other. The absorption tube is then weighed separately from the distilling bulb. The entire experiment may be recorded in table form as below.
Weight of distilling bvlb and eopper oxide before heating weight of distilling bulb and copper oxide after heating Weight of oxygen in water formed weight of absorption tube and eontents, stoppers (original) Final weight absorption tube, contents, and stopweight of water formed Percentare of orusen in water formed Perccntagc of hydrogen in water formed
...
Ezprrimcnl 2
Weiaht of distilling bvlb and copper oede before heating
~
~
..
.
44.48 g
.
Final weight abforption tibe. contents, and stoppers Weizht of water formed
0.46 g
