28 HYDRATES IN AQUEOUS SOLUTION. 



CALCULATION OF THE APPROXIMATE COMPOSITION OF THE HYDRATES. 



Given the above data, the calculation of the approximate amount of water 

 in combination with the dissolved substance is simple. The observed molec- 

 ular lowering is corrected for the difference between 1,000 grams and the 

 amount of water actually present in 1 liter of the solution. This gives the 

 true molecular lowering, produced by the substance at the dilution in ques- 

 tion, if there were 1,000 grams of water present. 



The calculated molecular lowering can now be compared directly with 

 the corrected, observed molecular lowering. If there was no hydration these 

 two values would be equal. The magnitude of the hydration is obtained as 

 follows : 



The calculated molecular lowering is divided by the corrected molecular 

 lowering found. This ratio, multiplied by 1,000, gives the amount of water 

 present playing the role of solvent, if the quantity of the substance present 

 was dissolved in 1,000 grams of water. 



The difference between this amount of water and 1,000 grams, is the 

 quantity that is in combination with the dissolved substance, under the 

 conditions that obtain in the solution in question. 



For a number of reasons everything is referred to 1,000 grams of the 

 solvent. The theoretical freezing-point constant is calculated for 1,000 

 grams of the solvent and, by calculating the depression found for the same 

 amount of the solvent, the two sets of results are comparable. Further, 

 by referring everything to a constant amount of the solvent, the various 

 results can be compared directly with one another. 



If we know the number of grams of water that have combined with the 

 dissolved substance, the number of gram-molecules of water that have 

 entered into combination is obtained by dividing the above value by 18. 

 If we divide this value by the concentration in terms of normal, we obtain 

 the number of molecules of water in combination with one molecule of the 

 dissolved substance, when the amount of substance present in a liter of the 

 solution is dissolved in 1,000 grams of water. 



THE RESULTS. 



In order to calculate the composition of the hydrates formed by any 

 given substance, at different dilutions, it is necessary to have the following 



values: 



The lowering of the freezing-point produced by the dissolved substance. 



