A STUDENTS' GAS-DENSITY BALANCE C. C. COFFIN Dalhousie University, Halifax, Canada
A
CCORDING to the gas laws the density of a gas
The large stopper carries a tightiitting 2" glass tube which eliminates enough rubber to prevent the stopper a t constant temperature is proportional to its pressure. The buoyant effect upon a body im- from being sucked in when there is a vacuum in the mersed in a gas is proportional to the density of the gas case. The float F is an evacuated 240-cc. Dumas bulb and therefore also to its pressure. Thus if two gases fused to the balance beam, which is constructed as a t different pressures have the same buoyant effect shown from light glass tubing. Two small brass bindupon a body their densities are equal and their molecu- ing posts B sealed into the ends of the glass crosspiece lar weights are inversely proportional to their respec- with de Khotinsky cement,carry the steel sewing tive pressures. An apparatus for determining the con- needles N which support the moving system. The ditions under which different gases have the same needle points bear upon a horizontal glass plate (lantern buoyant effect is known as a gas-density balance. slide cover glass) G stuck with melted wax to a block of par& wax W of such dimensions that when its edges rest on the sides of the balance case the crosspiece of the beam is approximately equidistant from the top and bottom of the case. A double-wing lock-nut L on a short length of threaded brass fixed into the glass pointer arm with de Khotinsky cement serves as an adjustable counter-weight to the float. The pointer swings over a vertical milk-glass scale S (from a broken thermometer) fastened with de Khotinsky cement a t its lower end to a glass rod'R, the other end of which is embedded in the par& block. This rod serves as a handle by which the balance may he slipped in or out of the case. The balance is assembled and adjusted outside the Such a balance constructed of standard materials case. The maximum sensitivity is of course obtained obtainable in any stock room has been found to add a by having the center of gravity of the moving system as rapid and instructive experiment to the ordinary labo- high as possible. It is surprisingly easy to approach ratory work in undergraduate physical chemistry. very closely to unstable equilibrium by simply adjustLongitudinal and cross-sections of the balance are ing the needles in the bindmg posts. A small brass shown in the accompanying diagram. The case is a connector on the upper part of one of the needles may bell jar 40 X 15 cm. (a large pyrex test-tube would be added as a convenient fme adjustment. By means serve as well) closed by a rubber stopper and connected of the glass rod R the paraffin block carrying the balby tubes T to a manometer, gas supply, and vacuum. ance is carefully pushed in until there is room for the
rubber stopper. A thermometer is placed inside so that its bulb is near the balance float and in such a position that it can be read from outside the case. The glass plate is leveled by turning the bell jar, the pointer is moved so that it will be close to but not touching the scale, and the stopper is inserted and pushed into place. When properly set up the system is so stable to ordinary treatment that it is very seldom necessary to remove the large stopper for readjustment-an important consideration in connection with leakage difficulties. Any minor derangement (side slipping of the needles, pointer fouling scale, etc.) can usually be corrected by tilting and tapping the case. The whole balance has on several occasions been carried from one table to another without the necessity of opening the case for readjustment. Dry dust-free air is of course used as the reference gas-the ratio of the density of the "unknown" gas to that of air being inversely proportional to the respective pressures a t which balance is obtained. In practice it is found convenient to determine the rest points (by the method of swings) in air a t several slightly different pressures not far from atmospheric. The
graph of rest point against pressure then gives a t once the pressure of air corresponding to the fmt rest point which happens to occur when the case is filled with the gas whose density is being determined. The use of a good manometer is justified in that the sensitivity of the balance is readily adjusted to well within one scale division (approximately 1mm.) per mm. of mercury. Lack of temperature control is the main source of error. This can be minimized by keeping the case covered during a series of measurements with a blanket of insulating material such as hair-felt. A serious error frequently arises from failure to flush out the balance case sufficiently with the gas being investigated. The purity and variety of the tank gases (Oz, COB, SOz, C2H2, NHs, etc.) now available afford the student a wide range of convenient material with which to work. A n instructive variation of the experiment is the analysis of two component mixtures. Students appear to find the simplicity and precision of the density balance rather refreshing after an experience with the laborious direct weighing method. The experiment affordsa good object lesson of the importance of correcting for air buoyancy in making absolute weighings.
