Validity and Limitations of Henry's Law - QS Study
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Validity and Limitations of Henry’s Law

Henry’s law is obeyed fairly satisfactorily by many gases of low solubility, provided the pressure is not too high or the temperature is not too low. This is because Henry’s law is intimately connected with ideal gas law. Henry’s law states that: At a constant temperature, the amount of a given gas dissolved in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid. Large deviations from Henry’s law are observed in the case of gases of high solubility and particularly those which interact with the solvent liquid, e.g., ammonia and hydrogen chloride gas in water. Ammonia forms ammonium hydroxide which partly dissociates into ammonium and hydroxyl ions while hydrogen chloride gas forms hydrogen and chloride ions in water.

Henry’s law is only valid for low dissolved gas concentrations. Henry’s law is a limiting law that only applies for “sufficiently dilute” solutions. The range of concentrations in which it applies becomes narrower the more the system diverges from ideal behavior.

Deviations from Henry’s law in these cases are attributed to change in the molecular species as result of dissolution. The law is strictly applicable to those gases where the molecular species are the same in the liquid phase. If suitable corrections are applied to account for such interactions, and concentrative of the same molecular species in the two phases are determined, Henry’s law might be applicable. This indeed was found to be true in the case of ammonia.