Thus nitric oxide is 15 times as heavy as hydrogen: how many times is it heavier than air? 15 × 06926 = 1.0389. Owing to their molecules being broken up by heat at temperatures but little above their boiling points, about fifteen substances form vapours having densities either half or two-thirds of the number obtained by halving their molecular weights. And in a few other cases the density of the vapour is expressed by the same number as the molecular weight. The more important of these exceptions are :— (18) VOLUME AND MASS OF GASES. It is found by experiment that 22:32 litres of any normal gas weigh a number of grams equal to the number express ing the molecular weight of the gas. Thus 22.32 litres of hydrogen (H1 = 2) weigh 2 grams. 2 This volume 22-32 litres is commonly spoken of as "two volumes" and expressed by the symbol . If great accuracy is not required, the more convenient number 22.4 litres may be used instead of 22-32 litres. Since 22.32 litres of any gas weigh its molecular weight in grams, a litre of any gas weighs its molecular weight in grams divided by 22.32, and one gram of any gas occupies 22.32 litres divided by its molecular weight. Hence the mass in grams of any volume of a gas can be found by multiplying the number of litres of it by its molecular weight and dividing by 22·32. Thus, to find the mass of 250 litres of chlorine (Cl=71), Conversely the volume in litres of any gas can be found by multiplying the number of grams of it by 22.32 and dividing by the molecular weight. Thus, to find the volume of 225 grams of hydrogen sulphide (H,S = 34), 2 Find the mass of 80 litres of oxygen (0,= 32) measured at 52° C. and 740 m.m. The gas must be reduced to 0° C. and 760 m.m. 80 × 273 740 32 8880 93.47 grams. What volume at 13° C. and 750 m.m. is occupied by 14-2 grams of chlorine (Cl, = 71)? (19) EQUATION AND VOLUMES OF GASES. When the volume of one gas is given and that of another gas is asked, since each molecular weight expresses 'two volumes' of the gas, the result may often be obtained directly. Thus, what volume of hydrogen chloride is formed when 10 litres of chlorine combine with hydrogen? = Two volumes of chlorine form twice two volumes of hydrogen chloride, hence 10 litres of chlorine form 2 × 10 20 litres of hydrogen chloride. If 10 litres of hydrogen at 15° C. are burnt, what volume of steam at 300° C. is formed? The volume of the steam would be equal to that of the hydrogen, if the temperatures were the same; making the correction for the change of temperature When however the mass of a solid or liquid is given or asked, and the volume of a gas is asked or given, the equation can only be solved in terms of the mass of the gas. Hence from the given volume the mass of the gas must be found by either of the last sections and the equation solved in the ordinary way. Thus, how much lead sulphide can be precipitated by 17 litres of hydrogen sulphide? H,S+ Pb2NO ̧ = 2HNO2+ PbS. How much phosphorus can be obtained by intensely heating 10 c.c. of hydrogen phosphide ? 10 c.c. of hydrogen phosphide weigh 01 x 17 x 0896 gms. 01 × 17 × 0896 × 2 × 31 2 x 34 = 01389 grams. Or if the volume of a gas is asked, the mass of the gas is found from the equation in the ordinary way, and its volume is then calculated by the last section. Thus, what volume of sulphur dioxide is formed on burning 8 grams of sulphur? 10 grams of carbon are burnt: what volume of carbon dioxide at 39° C. and 380 m.m. is formed? at 0° C. and 760 m. m., which become at 39° C. and 380 m.m. *(20) THE MASS OF A MOIST GAS. If the volume of a gas be measured, when it is saturated with the vapour of water, the pressure measured is due partly to the gas and partly to the steam. The tension of aqueous vapour (F) varies very rapidly with the temperature and must be found from tables (cf. Table IV.). |