TABLES OF THE RELATIVE DEPRESSION OF THE 



FREEZING POINT, 1860/A, TO FACILITATE THE 



CALCULATION OF MOLECULAR WEIGHTS 



J. ARTHUR HARRIS and ROSS AIKEN GORTNER 



(Station for Experimental Evolution, The Carnegie Institution of Washington) 



The molecular weight of a substance is given by C X K/ä^, where 



_ weight of solute - „ _ , , , , • r 



C = — ~, ? -, :, K = 1000 X the molecular lowering for a 



weight of solvent ^ 



given solvent, and A = the depression of the f reezing point in de- 



grees centigrade. While the arithmetic of these calculations is not 



onerous it is clear that tables for i^/A will be of considerable service. 



We have found them indispensable in dealing with large series of 



determinations of the mean molecular weight of the solutes in 



vegetable saps. 



The selection of the proper value from those which have been 



used for the molecular lowering, i. e., the depression of the freezing 



point produced by dissolving one mole of solute in 1000 grams of 



water, is the only point requiring consideration. We follow the 



more recent texts on physical chemistry in taking the value as 



roundly 1.86°.^ 



1 D. Berthelot's (Z. /. Elektrochemie, lo, 621-629; 1904) analysis of the ex- 

 perimental determinations results in 22.412 /. as the volume of a gram molecule 

 of gas under Standard conditions, 273.09° as the melting point of ice on the abso- 

 lute Scale, and 22.412/273.09 = 0.082086 =r i?, the gas constant. Taken in connec- 

 tion with the equation 



P = 12.06A — 0.021A2 



for the calculation of osmotic pressure from the depression of the freezing 

 point as used in our table [Harris, J. Arthur, and Gortner, Ross Aiken, Notes 

 on the Calculation of the Osmotic Pressure of Expressed Vegetable Saps from 

 the Depression of the Freezing Point with a Table of Values for P from 

 A=: 0.001° to Ar= 2.999° (in press)], and on the assumption that Van't Hoff's 

 law holds rigidly this gives (compare Lewis, G. N., /. Amer. Chem. Soc, 30, 

 670; 1908) 1.858° as the molecular lowering. In view of the widely varying prac- 

 tice of physical chemists we have, however, used the round number generally 

 employed, 1.86° instead of the theoretical 1.858°. Those who may desire for any 

 reason to use another value for the molecular lowering may do so by making 

 use of a factor, 



259 



