In vapor–liquid equilibrium for a dilute solute, Henry's Law relates the total pressure P, the vapor-phase mole fraction y_A, the liquid-phase mole fraction x_A, and the temperature-dependent Henry's constant H_A(T) by which equation?

• A. P y_A = x_A H_A(T)
• B. y_A = x_A H_A(T) / P
• C. P_A = H_A(T) / (x_A)
• D. P = x_A y_A / H_A(T)

Answer: (A)

Henry's law for a dilute solute says the partial pressure of A in the vapor, P_A, is proportional to its liquid mole fraction with the Henry constant: P_A = H_A(T) x_A. In vapor–liquid equilibrium, the partial pressure is also P_A = y_A P, since y_A is the mole fraction in the vapor and P is the total pressure. Setting these equal gives P y_A = H_A(T) x_A. This directly links the total pressure, the vapor-phase composition, the liquid-phase composition, and the temperature-dependent Henry constant. You can also rearrange it to y_A = [H_A(T) x_A]/P, which is the same relationship written differently. The other forms would not reflect the correct dependence established by Henry's law for a dilute solute.