Psychrometrics: The saturated molal absolute humidity of a vapour–gas mixture depends on which variables?

Introduction / Context:
Absolute humidity (on a molal basis) is central in humidification operations, dryer design, and environmental control. For saturated mixtures, the vapour partial pressure equals its saturation pressure, linking humidity directly to temperature and system pressure.

Given Data / Assumptions:

• Ideal gas behaviour for both carrier gas and vapour.
• Mixture is saturated at the dry-bulb temperature.
• Total system pressure P is uniform.

Concept / Approach:
At saturation, y_v = p_sat(T_db) / P. The molal absolute humidity (e.g., moles vapour per mole dry gas) is a function of the saturation vapour pressure at the prevailing temperature and the total pressure. Thus both the saturation pressure and total pressure determine the saturated composition.

Step-by-Step Solution:

Verification / Alternative check:
Psychrometric charts at different elevations (pressures) show different saturated humidity lines, confirming pressure dependence. At a fixed pressure, higher temperature increases p_sat and hence humidity.

Why Other Options Are Wrong:

• (a) only or (b) only ignores the explicit form y_v,sat = p_sat/P, which contains both variables.
• Neither: Incorrect because both variables are influential.

Common Pitfalls:
Assuming independence from pressure (valid only at very low y_v and narrow ranges); in general, pressure matters.

Final Answer:
both (a) and (b)