No. 20. EXPERIMENTS TO DETERMINE THE TENSION OF AQUEOUS VAPOUR IN PRESENCE OF A It was first necessary to make the mixture of known proportions of water and sulphuric acid. The method adopted was to determine the S. G. of the mixture, and hence get its per-centage composition from the tables. First a bottle of commercial "pure redistilled" acid was taken. This could not be tested by the S. G. method, since for mixtures from 95° to 100 per cent. acid there is no variation of density, and very little above 90 per cent. A portion of the commercial acid was poured into a flask holding about a litre. The flask and acid weighed 1124-373 grammes (all weights given in grammes). The acid was then poured off into a stoppered bottle destined to hold the mixture, and the emptied (but not dried) flask weighed 122-062 grams. This gives for the weight of the commercial acid 1002 311. The flask was then washed, and the same operation repeated with water, the water being poured into the same stoppered bottle. As the water had to be poured in by very small quantities at a time (owing to the heat developed), there was a certain loss from drops that ran down the outside of the flask after each pouring. An idea of the amount of water thus lost was got by standing the flask on a glass plate, which was weighed with it before and after, but this would not account for the whole loss, owing to evaporation. This gives for the weight of the water 544-500 (In all dealings with the acid, care was taken to exclude the air as much as possible.) There was then in the bottle a mixture, which we call Mixture I. as follows: The method adopted for determining the specific gravity was by means of a glass bob, loaded with mercury, and hung by a platinum wire to the scale of a balance. A rather thick wire was used to avoid the danger of breakage. No error is introduced on account of the weight of the wire, if care is taken to have it always immersed to the same depth. This was got by using the same beaker for all the weighings, a mark on it shewing the proper level of the liquid. What we want to obtain is the specific gravity of the various mixtures of water and acid, i.e. the ratio of the weight of a volume of the mixture at 15° to that of the same volume of water at 4°. Let weight of bob in air = W, " mixture at t2 = W", specific gravity of water at t1° = d1 (compared with water at 4°), (compared with mixture at 15°). mixture at t2° = d2 (The specific gravity of all these mixtures is taken at 15°, compared to water at 4°, in Kohlrausch.) Then specific gravity of mixture = W-W" d 1+k (t, − 15) where k = coefficient of expansion of According to Kohlrausch k=00016 +00001 × p where p is the percentage of pure acid in the mixture. Hence specific gravity of mixture For this formula, p need only be known approximately. (weight of bob in air (W) 26·0356, water (W') 16.6367, temp. 116, Mixture I. (W") 11.6835, temp. 13.5. W-W' 9.3989 = This corresponds to a strength of 62 per cent. Hence the 1546.811 grammes contain 959 023 of pure acid. Hence the 1002:311 grammes of commercial acid contain this quantity of pure acid. This gives for the percentage composition of the commercial acid 95.68 per cent., H.SO. In order to take the weighing, a portion of mixture (I.) had to be poured out into a beaker. There was not enough of the mixture for this portion to be wasted, so the following method was adopted: The empty beaker was weighed (a). Then the beaker with the portion poured out (B). Then, after the specific gravity weighing (which involved inserting the bob and the thermometer) the weight was taken again (y). Then the mixture was poured back into the bottle, and the emptied beaker weighed (8); (Ba) is the weight poured out, (d) is the weight poured back; (B-a-y+8) is the weight lost in the observation, This quantity gives (Commercial acid 840) Mixture I. in the bottle then consists of: Comm. acid 1001-471 = 1.302. 462 There is now added to the above more water, weighed in the same manner as previously described. Weight of water added 275.739. |