Question

when will the Henry's law is the special case for the raoults law

Answer 1

The Henry's law is given by: 


where p is the partial pressure and x is the mole fraction. 
kH is the proportionality constant (Henry's constant)

The Raoul'ts law is given by: 


where p is the partial pressure and x is the mole fraction.
p* is the vapor pressure of the pure component. 

So, Raoul'ts law is a special case of Henry's law when kH = p* . 
The Henry's constant is approximately equal to the vapor pressure only for 'ideal' mixtures (mixtures obeying the Raoult's law over the entire range of composition) which doesn't entirely exist but closely related substances like benzene and toluene can form a close-to-ideal-mixture where this case applies. 

In real-time scenario, the Raoult's law and the Henry's law are obeyed in opposite composition ranges. That is probably why, it's Raoul'ts law is a "special" case of Henry's law.

Answer 2

Answer:

Henry's law is one of the gas laws states that: at a constant temperature, the amount of a given gas that dissolves in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid.

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