If M' be the molecular weight of the solvent employed, then T = M' X 063 very nearly, with the exception of the solvent water. Thas been determined by numerous experiments for the commoner solvents; its mean value for: = 70'7. 18.5 (for organic substances, some salts of diad metals, all the feeble bases and acids). 37 (for alkaline and alkaline earthy salts, and all the strong acids and bases). EXAMPLE. The freezing-point of a sample of acetic acid was found to be 16490° C.; taking 62'014 grams of this acid and - adding thereto 0'2540 gram of pure propionic acid, the solidifying point of the mixture is found to be 16.277° C. What is the molecular weight of propionic acid? The observed lowering of the freezing-point is 16.490 16'277 = 0'213° = K, the weight of solvent P=62'014 gram, weight of dissolved body pi O'2540 gram, and 7' = 38'6 for acetic acid, hence = (Calculated molecular weight of C2H,COOH = 74) QUESTIONS. 266. If the molecular lowering of the freezing-point of acetic acid, 16°75° C., be 43 in the case of benzoic acid, what is the temperature of solidification of a 4 per cent. solution? 267. Plot out a curve showing the relation between concentration and K in the case of ethyl formate dissolved in acetic acid, and find the mean molecular weight of the ethyl formate as given by this series of experiments. 268. Find the coefficient of lowering of the freezingpoint of acetic acid by ethyl formate from the data of the preceding question. 269. What percentage of methyl acetate has been added to a sample of acetic acid freezing originally at 16·52° C. and after addition of the ester at 15'619° C. ? 270. By what amounts should the freezing-point of benzene be lowered in 5 per cent. solutions of each of the following bodies-toluene, xylene, and anthracene? 271. Taking 567 gram of acetic acid freezing at 16.52° C. and adding methyl acetate in successive quantities as follows: (1) o'0725 gram, (2) o'0970 gram, (3) 00821 gram, (4) o'1192 gram, the freezing-points after each addition are observed to be (1) 16°452°C.,(2) 16·363°C., (3) 16·282° C., and (4) 16°177° C. respectively; what is the molecular weight of methyl acetate found from each observation? 272. What should be the normal molecular lowering of the freezing-point in the case of (a) anthracene, (b) sodium, (c) tin? 273. 6 grams of anhydrous magnesium sulphate are dissolved in 100 grams of water; the observed depression of the freezing-point is o'958° C. Taking the value of T to be 37, which of the following formulæ most probably represents the state of the dissolved salt, (a) MgSO4: (6) MgSO4, 7H2O? 274. It has been found that the latent heat of fusion of a solvent is connected with the molecular lowering of its freezing-point as shown in the formula, The freezing-point of ethylene dibromide being 7.9° C., and the molecular depression of its freezing-point 1179, find its latent heat of fusion. 275. The value of T for water is 18.5, for formic acid 277, and for acetic acid 386, when organic substances are dissolved in each solvent. What inference may be drawn as to the relative complexity of the molecules of each of these solvents? 276. What conclusion would you draw as to the molecular weight of nitrogen tetroxide from the following data? (a) o'5269 gram of CHCl, introduced into 17.3046 grams of nitrogen tetroxide lowers the freezing-point of the latter by 1'06°. (6) Similarly o'4931 gram of CHCl mixed with 16.7650 grams of nitrogen tetroxide lowers its freezing-point I'09°. 277. Find the molecular weight of gold, from the figures below showing the effect of Au in lowering the freezing-point of Na. (Take T = 104). 278. Plot out a curve to show the variation of the molecular lowering of the freezing-point with concentration, using the data obtained by Haycock for mercury dissolved in sodium. THE capacity of a body for heat is measured by determining the number of units of heat required to raise that body one degree of temperature. The Specific Heat of a body is the ratio of the quantity of heat required to raise that body one degree to the quantity required to raise an equal weight of water one degree. Latent Heat is the quantity of heat which must be communicated to a body in a given state in order to convert it into another state without changing its temperature. The Atomic Heat of an element is the product of its atomic weight into its specific heat; for the greater number of the elements, the mean value of this quantity is 6'40. This is expressed by Dulong and Petit in the form of a law, thus :-The atoms of all elementary bodies have exactly the same capacity for heat. The following table by Regnault shows the specific heat of the more important elements and compound gases between 0°-100° C. The following table, embodying the results obtained by Person, gives the latent heat of several bodies : Water Chlorine O'1214 Carbon dioxide . 0'2164 Nitric oxide . 0*2238 Steam 0'475 The heat required to raise the temperature of a kilogram of water through 1° C. is able to do work equivalent to the lifting of a kilogram weight through 425 metres. The mechanical equivalent of heat is 425 metre-kilograms. A kilogram of water falling through a height of 425 metres, and having its motion suddenly arrested, would have its temperature raised 1° C. |