MOLECULES AND ATOMS 985 



show (according to Pickering's data 1892) for solutions of NaCl and 



CuS0 4 in water. For 



w = 0-01 0-03 0-05 0-1 0-5 



molecules of NaCl the depression is 



rf = 0-0177 0-0598 0-0992 0-1958 0<"9544 



which corresponds to a depression per molecule 



K=l-77 1-96 1-98 1-96 1-91 



i.e. here in the most dilute solutions (when n is nearly 0) d is obtained 



about t'7 n, while in the case of sugar it was about I'l n. For CuSO 4 



for the same values of n, experiment gave : 



<J=0-0164 0-0451 0-0621 0-1321 _ 0-5245 

 K= 1-64 1-50 1-44 1-32 1-05 



i.e. here again d for very dilute solutions is nearly 17 n, but the value 

 of K falls as the solution becomes more concentrated, while for NaCl it 

 at first increased and only fell for the more concentrated solutions. 

 The value of K in the solution of n molecules of a body in 100H 2 O, 

 when d = Kra, for very dilute solutions of CaCl* 3 is nearly 2'6, for 

 Ca(NO 3 ) 2 nearly, 2-5, for HNO 3 , KI and KHO nearly 1 -92-0, for 

 borax Na 2 B 4 O 7 nearly 3-7, &c., while for sugar and similar substances 

 it is, as has been already mentioned, nearly TO I'l. Although these 

 figures are very different 28bi8 still k and K may be considered constant 

 for analogous substances, and therefore the weight of the molecule 

 of the body in solution can be found from d. And as the vapour 

 tension of solutions and their boiling points (see Note 27 bis and 

 Chapter I., Note 51) vary in the same manner as the freezing point 

 depression, so they also may serve as means for determining the mole- 

 cular weight of a substance in solution. 29 



Thus not only in vapours and gases, but also in dilute solutions of 

 solid and liquid substances, we see that if not all, still many properties 



is bis Their variance is expressed in the same manner as was done by Van't Hofl 

 (Chapter I., Notes 19 and 49) by the quantity i, taking it as =1 when fc = l'05, in that case 

 for KI, i is nearly 2, for borax about 4, &c. 



2 9 We will cite one more example, showing the direct dependence of the properties of 

 a substance on the molecular weight. If one molecular part by weight of the various 

 chlorides for instance, of sodium, calcium, barium, &c. be dissolved in 200 molecular 

 parts by weight of water (for instance, in 8,600 grams) then it is found that the greater 

 the molecular weight of the salt dissolved; the greater is the specific graviiy of the 

 resultant solution. 



Sp. gr. at 16 



1-0328 

 1-0331 

 1-0489 



