Page:Text-book of Electrochemistry.djvu/69

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��BOILING AND FREEZING POINT.

��CHAP.

��it gives correct values for a solution of alcohol in water, although the alcohol has a much higher vapour tension than the water. Furthermore, the freezing point method gives much more exact values than the boiling point method. This latter is consequently chiefly used for the determination of the molecular weight in cases where the freezing point of the solvent, e.g, alcohol, ether, carbon disulphide, can only be reached^with diflBculty.

Connection between Depression of Vapour Pressure and Depression of Freezing Point. — In 1870, Guldberg (i) proved theoretically that the vapour pressure and the freezing point stand in close relationship, so that the two corresponding depressions run parallel; and he further showed that this was confirmed by experiment. On the basis of purely experimental data, Eaoult, in 1878, again brought forward this same statement, and showed that it applied to one per cent, salt solutions. As the numbers obtained by Eaoult possess a certain historical interest, they are reproduced in the following table : —

�� �Lowering of the

�Salt.

� �FreezlDg point.

�Vaponr preesnre in mm. iJg at 100°.

� �i Degree.

� �Mercuric chloride ....

�(»-04H

�005« X 7G

�Mercnric cyanide.

�0-087 „

�Lead nitrate

�0-110 ;,

�Barium nitrate.

�0-137 ,.

�Silver nitrate.

� � �(M4G

�0-1(50 ;.

�Potassium ferricyanide

� � �' 0-14<>

�0165 ,,

�Potassium chromate

� � �. ! -20

�0-213 „

�Potassium sulphate Potassium iodide.

�0-201 „

�0-225 „

�Potassium cldorate

�0-240 .,

�Potassium nitrate.

�0-280 „

�Ammonium sulphate

�0-230 ;,

�Potassium bromide

�0-310 ,,

�Sodium nitrate. .

�0-380 „

�Ammonium nitrate

�0-361 „

�Potassium chloride

�0-450 „

�Sodium chloride.

�0-604 .,

�Ammonium chloride

� � �0G3J)

�0-565 ;,

��The proportionality is not so good as perhaps might be

�� �