Air at 40°C has partial vapour pressure 2.4 kPa. Universal gas constant 8.314 kJ kg-mole⁻¹K⁻¹, total pressure 101.325 kPa. Humid volume of air in m³ (kg dry air)⁻¹ is
Options:
(A) 0.809
(B) 0.908
(C) 1.089
(D) 1.098
Official Answer Key:
B
Step-by-Step Solution & Explanation
Humidity ratio Y = 0.622·P_v/(P−P_v) = 0.622×2.4/(101.325−2.4) = 0.01509 kg water/kg dry air. Humid volume V_h = (R·T/P)·(1/M_air + Y/M_water) = (8.314×313.15/101.325)×(1/28.97 + 0.01509/18.02) = 0.908 m³/kg dry air.
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