Humidity terminology: The ratio of the existing moles of vapor per mole of vapor-free gas to the moles that would be present if the mixture were saturated at the same T and P is called

Introduction / Context:
In gas–vapor systems (such as air–water vapor), several saturation measures are used in mass-transfer and drying calculations. Distinguishing relative humidity, relative saturation, and percentage saturation avoids confusion and ensures correct graphical and computational use of psychrometric charts.

Given Data / Assumptions:

• Gas–vapor mixture at specified temperature and pressure.
• Vapor is the condensable species; the carrier gas is non-condensable.
• Definitions based on molar ratios (vapor per mole of dry gas).

Concept / Approach:
Relative humidity (on a mole ratio basis) is defined as RH = (y / y_s), where y is the actual moles of vapor per mole of vapor-free gas and y_s is the saturation moles per mole of vapor-free gas at the same temperature and pressure. By contrast, relative saturation and percentage saturation involve ratios based on partial pressures and include corrections for total pressure and non-idealities; these often differ numerically from simple RH, especially at higher humidities or pressures.

Step-by-Step Solution:

Verification / Alternative check:
Psychrometric relations show RH equals the ratio of actual to saturation partial pressures under ideal gas assumptions; since y ∝ p_v/(P − p_v), using the same T, P definition recovers the mole-ratio form given above.

Why Other Options Are Wrong:

• Relative saturation and percentage saturation use different denominators or correction factors; they are not the simple mole-ratio definition cited.
• None of these is invalid because the described ratio matches the standard RH definition.
• Humid volume is a property (mixture volume per mass of dry gas), not a saturation ratio.

Common Pitfalls:
Interchanging RH with percentage saturation; always check whether the ratio is defined on a mole basis and whether corrections for total pressure are included.

Final Answer:
relative humidity