Henry s Law can be written for other concentration units. Make sure you check the units of K to know how you should specify solute concentration. Deviations from Ideal Behavior Real solutions give partial pressures that do not obey Raoult s Law. The observed partial pressures may be greater than or less than the prediction of Raoult s Law. Positive deviations indicate repulsive solvent-solute interactions relative to an ideal solution.
Negative deviations indicate attractive solvent-solute interactions relative to an ideal solution.
We will now develop a more quantitative, thermodynamic description of ideal solution behavior and deviations from that behavior. The P i in a real solution deviate from ideal behavior. To account for this, we define the thermodynamic activity of substance i as and the expression for the chemical potential becomes Thermodynamic activity coefficient We may divide the two definitions to give the thermodynamic activity coefficient of i , or The activity coefficient thus expresses the deviation of real behavior from ideal behavior. In many cases, g is all we need to know about a substance in solution. We are using subscript 2 here to emphasize that this convention applies to solute only.