[gear2d]

I have a question about the freezing point depression (Delta T = Kb * m * i) and boiling point depression (Delta T = Kf * m * i). I know that "i" is a colligative property (changes with number and not the type of substance used). So if I have "i" = 2 that means in freezing point depression the temperature will decrease by 2 units (say my solution has an original freezing point of 20 degrees, now it will have it at 18 degrees). As where in boiling point depression it will increase by 2 units. But happens to the vapor pressure in each case? Also, will adding a non-volatile substance, or a volatile substance change delta T?
 


Thank You in advance

[tamim83]

The vapor pressure will also be lowered by a factor of i.  The vapor pressure lowering is given by

delta P = iP^o_AX_B

where P^o_A is the vapor pressure of the pure A and X_B is the mole fraction of the ionic solute.