Moist-air mixtures: “The total pressure of moist air equals the sum of the partial pressure of dry air and the partial pressure of water vapour.” This statement is known as:

Introduction / Context:
Psychrometrics treats moist air as an ideal mixture of dry air and water vapour. The relationship between total pressure and constituent partial pressures underpins property calculations and chart construction.

Given Data / Assumptions:

• Ideal-gas behavior for each component at HVAC conditions.
• Mixture of dry air and water vapour in equilibrium at a given temperature and total pressure.

Concept / Approach:
Dalton’s law states that the total pressure of a gaseous mixture equals the sum of the partial pressures of the components, each evaluated as if it alone occupied the entire volume at the mixture temperature.

Step-by-Step Solution:
Let p = total pressure; p_a = partial pressure of dry air; p_v = partial pressure of water vapour.By Dalton: p = p_a + p_v.Use ideal-gas relations for each component to compute humidity ratio and relative humidity as needed in HVAC analysis.

Verification / Alternative check:
Compare computed humidity ratio from p_v with chart values; they match when Dalton’s law holds (typical at low to moderate pressures).

Why Other Options Are Wrong:

• Kinetic theory and Avogadro’s hypothesis are foundational but do not directly state this additive pressure law; Newton’s law is unrelated; Clapeyron’s equation is a thermodynamic relation for pure substances.

Common Pitfalls:
Applying Dalton’s law at very high pressures where real-gas effects are significant; in HVAC ranges, the ideal assumption is valid.

Final Answer:
Dalton’s law of partial pressures