W. G. Breck and F. w. Holmes Queen's University Kingston, Ontario, Canada
II
An Experimental Approach to the Ideal Gar L m
It is possible to introduce to high school students the equation of state, PV = nRT, by an experimental method which demonstrates that the quantity P V / T is a constant characteristic of a given amount of gas. Simple apparatus for this purpose is shown in the figure.
length of mercury below the enclosed gas.' To also include a Charles' law effect, the tube is enclosed by a large, stoppered test tube containing water and a, thermometer. This assembly can be put in a suitable bath to bring it to a desired temperature as read on the thermometer. Some actual readings made a t the three levels of pressure and two levels of temperature are given in the table. It can be seen that P V ' / T (and hence P V I T ) is constant for the given amount of enclosed gas within reasonable limits of experimental error. Data for Various Pressures and Tem~eratures.
Apporatvr for PV/T determination
A capillary tube, about 1 mm inside diameter and sealed a t one end, has in it a length of mercury which thus entraps a quantity of air. When the tube is horizontal the enclosed gas is taken a t atmospheric pressure; when the tube is vertically up the pressure, P, is more than atmospheric by the length of mercury above the enclosed gas; when the tube is vertically down the pressure is less than atmospheric by the ' I t is not our intention to claim credit for this ingenious Boyle's law arrangement the ultimate origin of which is not known and might be of interest. It has been in use by teachers in Kingston, Canada, for many years and was suggested as a trial demonstration in the CBA program. EDITOX'sNOTE: We invite correspondence with readers who have knowledge about the origin of this suggested apparatus. See also THIS JOURNAL^^, .534 (1955).
It would be advantageous to have a number of such experiments set up in the same laboratory to demonstrate that for each given amount of enclosed gas P V / T is constant but for different amounts of enclosed gas P V / T diiers. I n this way it can be shown expem'mentally that the amount of enclosed gas is characterized by the valne of P V / T . If now, for 1 mole of gas a t STP, V is taken as 22.4 1, and P V / T as R , then for n moles V = 22.4 n liter, and P V / T = nR.
Volume 44, Number 5, Moy 1967 / 293
