Page:Text-book of Electrochemistry.djvu/62

 CHAPTER V.

��Boiling Point and Freezing Point of Solutions.

��Calculation of the Boiling Point of a Solution*— The

curve pf (Fig. 14) represents the change of vapour pressure of water (or other solvent) with temperature near the boiling point (at 760 mm.) ; the curve j!>y represents in the same way the vapour pressure of a solution in the same solvent, which, according to what has been stated above, must be lower than that of the pure solvent. The boiling point of the solution (at 760 mm.) is found by drawing through A, which lies on jpp- directly above Ty a line parallel to the ab- scissa-axis. This horizontal line corresponds with a pressure of 760 mm., and cuts the curve 'pp at B, A line is then dropped perpendicularly from B^ cutting the abscissa-axis at J?, or jT-I- dT. The perpendicular cuts />/> at C> which corresponds with a pressure of 760 mm. -h dp. If the inclination a of the curve at A is known, we can find

dp tana = ^f

The part AC ^i the curve may be regarded as a straight line, and we then have —

CB^p^l) =^ (^r^lT = dT tan a.

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