Saturated gas mixtures: For a gas saturated with a condensable vapour under ideal-gas assumptions, the volumetric (mole) fraction of vapour depends on vapour saturation pressure and total pressure. It is independent of which factor?

Introduction / Context:
Humidification and gas–vapour contacting rely on saturated-gas relationships. For an ideal saturated mixture, the vapour mole fraction is given by y_v = p_sat(T) / P_total. This allows quick estimation of humidity and dew-point behavior.

Given Data / Assumptions:

• Ideal-gas behavior for both vapour and carrier gas.
• Gas is saturated with the vapour at temperature T.
• Vapour partial pressure equals its saturation pressure p_sat(T).

Concept / Approach:
At saturation, y_v = p_v / P = p_sat(T) / P. Therefore, y_v depends on temperature (through p_sat) and on total pressure P. It also depends on the liquid identity because different liquids have different p_sat(T). However, the identity of the noncondensable gas does not appear in y_v for ideal systems.

Step-by-Step Solution:

Verification / Alternative check:
Psychrometric parallels show humidity ratio depends on p_sat(T) and pressure; whether the carrier is air, nitrogen, or helium does not change y_v ideally.

Why Other Options Are Wrong:

• Nature of the liquid: Determines p_sat(T); crucial.
• Temperature: Controls p_sat; crucial.
• Total pressure: Explicit in y_v; crucial.

Common Pitfalls:
Non-ideality (solubility, association, or high pressures) can introduce dependence on gas identity; this question assumes ideal conditions.

Final Answer:
Nature of the noncondensable gas