The activity coefficient in relation to Raoults and Henrys laws

The activity coefficient in relation to Raoult s and Henry s laws 

For a non-ideal liquid mixture in contact with a perfect vapour phase we obtain, as the condition of equilibrium, in place of (8 16), 

If we choose to approach unity as Xi approaches unity—as on Convention I for all species and on Convention II for the solvent— then Ki must clearly be the same aep, the vapour pressure of the pure component at the temperature and total pressure of the solution. Thus (9 4) may be written 

The significance of y may perhaps be seen more clearly from Fig. 38, which shows typical vapour-pressure curves of a binary mixture of components A and B, Consider the particular composition corresponding to a point R. If the solution were ideal the partial pressure of B above the solution would have the value which is represented by the length BT. The actual partial pressure is represented by the length BS. Therefore firom equation (9 6) we have 

In brief, whenever we choose 1 as 1 the activity coet cients have a simple interpretation as the ratio of the actual partial pressure to the partial pressure which would occur if Raoult s law were obeyed. One of the most direct methods of determining these coefficients is by measurement of the partial pressures. 

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