Raoult’s law for volatile solutes states that in a solution, the vapour pressure of a component at a given temperature is equal to the mole fraction of that component in the solution multiplied by the vapour pressure of that component in the pure state.
The vapour pressure P of a solution containing two components A and B is
PA = PoA × XA
PB = P0B × XB
P = PA + PB = PoA × XA + P0B × XB
where, is the vapour pressure of component A
is the vapour pressure of component B
is the vapour pressure of component A in its pure state
is the vapour pressure of component B in its pure state
is the mole fraction of component A
is the mole fraction of component B
Now we know that,
XA + XB = 1
XA = 1- XB
P = (1-XB)P0A + XBP0B
P = P0A + XB(PB0 – P0A)
For a solution to be able to obey Raoult’s law at all concentrations, it should be an ideal solution. For a solution to be called ideal, it should have the following two characteristics:
•The enthalpy of mixing of the pure components to form the solution should be zero, that is, . It means that no heat is absorbed or released during mixing.
•The volume of mixing the pure components should be zero, that is, . This means that the volume of the solution would be equal to the sum of volumes of the two components.
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