REACTIONS IN HOMOGENEOUS SYSTEMS 31 



The decrease of the vapour pressure is therefore propor- 

 tional to the osmotic pressure as both depend upon the number 

 of molecules in the solution. The decrease of the vapour 

 pressure is difficult to measure, but the rise of boiling point 

 or fall of freezing point can be used to calculate the osmotic 

 pressure. 



By measuring the vapour pressure of a pure substance at 

 different temperatures and by plotting the results so that the 

 abscissae represent degrees Centigrade and the ordinates the 

 vapour pressure in millimetres of mercury pressure, a tempera- 

 ture-pressure diagram is produced. Below the freezing point 

 the curve falls more steeply and represents the vapour pressure 

 of the solidified solvent. When the vapour pressure of a 

 liquid is equal to the pressure of the atmosphere the solution 

 boils. 



If on the diagram representing the vapour pressure curve 

 of water we draw a curve of vapour pressure of a solution we 

 see that at any given temperature the vapour pressure of the 

 solution is always less than the vapour pressure of the pure 

 solvent. The curve for the solution cuts the curve of the 

 solidified solvent at a temperature below the freezing point 

 of the solvent ; this represents the freezing point of the solu- 

 tion. The decrease of vapour pressure depends upon the 

 concentration of the solution and the point at which the 

 vapour pressure curve of the solution cuts the vapour pressure 

 curve of the solidified solvent depends on the decrease of the 

 vapour pressure, hence the depression of the freezing point 

 can be used to calculate the osmotic pressure (molecular 

 concentration) of the solution. 



On following the vapour pressure curve upwards it is seen 

 that in order to raise the vapour pressure of the solution until 

 it is equal to the pressure of the atmosphere, the temperature 

 must be higher than that which would raise the vapour pressure 

 of the pure solvent to the same pressure : that is the boiling 

 point is raised. As the rise in the boiling point depends upon 

 the concentration of the solution, the rise can be used to 

 calculate the osmotic pressure of the solution. 



CALCULATION OF MOLECULAR CONCENTRATION OF A 

 SOLUTION FROM THE FREEZING POINT DETERMINATION 



For any solvent the freezing point and boiling point are 

 altered to a known extent for corresponding concentrations. 

 In water a solution containing one molecular weight in grams 

 in a litre of solution decreases the freezing point by i-86C, 



