When does Raoult's law become a special case of Henry's law?

Raoult's law and Henry's law are both related to ideal solutions, but they apply to different scenarios.

Raoult's law primarily deals with ideal solutions of volatile solutes in solvents, stating that the partial pressure of each component of an ideal mixture of liquids is equal to the vapor pressure of the pure component multiplied by its mole fraction in the mixture.

Henry's law, on the other hand, deals with the solubility of a gas in a liquid. It 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.

Henry's law becomes a special case of Raoult's law when the solute (gas) is present in low concentration, such that it doesn't significantly affect the vapor pressure of the solvent. In this case, the vapor pressure of the solvent is essentially the same as the vapor pressure of the pure solvent, and the concentration of the solute in the solution is low enough to not affect this equilibrium.

So, in summary, Henry's law becomes a special case of Raoult's law when the solute concentration is low, and its effect on the vapor pressure of the solvent can be neglected.