If the determination be made at ° C., then Rel. dens. = W x rel. dens. of water at ° C. EXAMPLE. A piece of iron sulphide (white pyrites) weighed in air 48934 grams in water it weighed 3.8860 grams. Calcu II. The specific gravity flask method. If W = weight of substance in air, w' flask filled with water, w" = = weight of weight of flask filled with substance and water; we have weight of substance in water = w" w' and hence 2. The relative density of a liquid is commonly found by : : I. The specific gravity flask method. weight of flask weight of flask filled with Let x = weight of the empty flask, w = filled with water at t° C., w' liquid under examination, then = x rel. dens. of water at t° C. II. Weighing an insoluble solid in water and in the liquid. Let x = weight of solid in air, w = weight in water at to C., w' weight in liquid, then 1. Find the rel. dens. of absolute alcohol from the 2. A glass rod, weighing 13 grams in air, weighs 8 grams in water, and 38 grams in sulphuric acid; what is the sp. gr. of the sulphuric acid? Density of gases and vapours:-The density of any substance at a known temperature (and pressure, in the case of gases and vapours) is the mass of a unit of volume of the substance at that temperature (and pressure). The sp. gr. or rel. dens. of a gas or vapour is the ratio of its mass to the mass of an equal volume of hydrogen measured at the same temperature and pressure. The rel. dens. of a gas is commonly termed the density of the gas, and the rel. dens. of the vapour of any substance the vapour density of the substance. One gram of hydrogen under standard conditions measures 11.1636 litres; this volume accordingly becomes a convenient unit of volume for measuring densities. Using this unit, the densities are expressed by the same numbers as the relative densities or specific gravities. One litre of hydrogen gas at o° C. and 760 mm. barometric pressure and at the level of the sea and latitude 45° weighs o'08958 gram. 3. The rel. dens. of a gas is determined by : I. Weighing a known volume of the gas by means of :— A. A counterbalanced globe, filled by the evacuation method. B. The displacement method: a bulb of known volume containing an inert gas is kept at a constant temperature while the experimental gas is passed in through one of the leading tubes in such a way as, so far as possible, to fill the bulb by the displacement of the contained gas. C. The method of collecting in a light small flask over mercury (Bunsen). II. The effusion method (Bunsen). The rel. dens. varies directly as the square of the time of effusion of equal volumes. EXAMPLE. In one of Bunsen's experiments, a certain volume of air escaped under pressure through a minute orifice in 1027 seconds; under exactly similar conditions, the same volume of CO, required 1270 seconds to escape through the same orifice. What is the rel. dens. of CO2 (air = 1)? Rel. dens. of CO2 Rel. dens. of air = 127.02 16129 = 102.72 10547 dens. of CO2. = I'5292 = Rel. 4. The rel. dens. of a vapour is found usually by : I. A. The direct measurement of the volume of vapour produced by the evaporation in an enclosed space of a known weight of the substance the principle of the methods of Gay Lussac and Hofmann. B. The indirect measurement of the same quantity, the volume of an inert gas displaced being actually measured -the principle of Victor Meyer's method. Or II. The weighing of a known volume of the vapour taken at an ascertained temperature and pressure-the methods of Dumas, and Deville and Troost. If D = the required density of the vapour referred to H as unity, we have in case I. A. D = W = H= h = = w.760. (1+0.00367 T) v. 000008958. (H − h). (1 + kT)' where weight of liquid taken, and hence weight of vapour formed. observed volume of vapour in c.c. reduced height of bar. at time of experiment. reduced height of mercury in tube above that in cistern. T = temperature of vapour. coefficient of cubical expansion of glass for 1° C. ข H p T = w.760. (I + 0·00367 T) v.0*00008958. (H − p) weight of liquid taken. the observed volume of displaced gas in c.c. vapour tension of water at temperature of temperature of water in collecting trough. The methods under 4. II. for determining vapour density and under 3. I. for finding the rel. dens. of a gas require the following data : the weight of bulb in air empty. the weight of bulb filled with gas or vapour in air. the capacity of bulb in c.c. the volume of the residual air in c.c. height of bar. and the temp. at which P' is found. height of bar. and T H' = k 0'00367 = = the reduced temp. of the bath at the time of sealing or closing the bulb. the coefficient of cubical expansion of the material of the bulb. the coefficient of expansion at constant pressure of a gas. vapour taken for (V v) is the volume occupied by the air displaced by the vapour or gas and the weight of the displaced air = 0'0012932. (V – v) H (1 + 0·00367. t). 760; (As v is generally small, any small differences between the pressure and temperature under which it is measured and H and t may be neglected.) and the volume occupied by this weight at H' and T is hence the rel. dens. referred to air as unity is given by if hydrogen be taken as unity o'0012932 is replaced by 0'00008958 the weight of 1 c.c. of hydrogen in grams. |