Henry's Law for NEET: Key Points, Tricks & MCQs

What NEET Asks

Conceptual questions: Direct statement, factors affecting solubility of gases, applications in daily life and biology.
Numerical problems: Calculating partial pressure or mole fraction of a gas in solution.
Weightage: Moderate, often appears as part of a larger problem on solutions or directly as a single MCQ.

Key Points

Statement: Henry's Law states that the partial pressure of the gas ( $P_A$ ) in the vapor phase is directly proportional to the mole fraction of the gas ( $x_A$ ) in the solution.
Mathematical Form: $P_A = K_H \cdot x_A$ , where is Henry's Law constant.
Solubility Relation: Higher $K_H$ at a given temperature implies lower solubility of the gas in the liquid.
Temperature Effect: The solubility of gases in liquids always decreases with increasing temperature (exothermic dissolution).
Applications: Carbonated beverages (CO $_2$ solubility), deep-sea diving (N $_2$ solubility and decompression sickness), high-altitude sickness (O $_2$ solubility in blood).
Limitations: Applicable for dilute solutions, low gas pressures, and gases that do not react chemically with the solvent or undergo dissociation/association.

Must-Know Formula / Reaction

$P_A = K_H \cdot x_A$

$P_A$ : Partial pressure of the gas above the solution.
$K_H$ : Henry's Law constant (units are same as pressure, e.g., atm, bar).
$x_A$ : Mole fraction of the gas in the solution (dimensionless).

Common Mistakes

Students often confuse the direct or inverse proportionality between $K_H$ and solubility. Remember: higher $K_H$ = lower solubility.
Don't apply Henry's Law to gases that react chemically with the solvent (e.g., NH $_3$ or HCl in water) or when concentrations are high.

Rapid Revision

Henry's Law ( $P_A = K_H \cdot x_A$ ) quantifies gas solubility: partial pressure directly relates to mole fraction. High means low solubility. Remember its applications in drinks, diving, and altitude, and its limitations (dilute, non-reactive gases, low pressure).