Question

The vapour pressure of water at 20 °C is 17.5 torr. What will be the no. of moles of water present in one litre of air at 20 °C and 40% relative humidity. (R =0.082 L atm/mol-k)

• A. (A) 4.2 × 10–4 mole
• B. (B) 4.2 × 10–6 mole
• C. (C) 3.82 × 10–4 mole
• D. (D) 3.82 × 10–6 mole

Answer 1

Answer: (C)

(C) 3.82 × 10–4 mole

Answer 2

Explanation:
Relative humidity (RH) = Partial pressure of H2O in air Vapour pressure of H2O∴ Partial pressure of H2O=RH× Vapour pressure of H2O=40100×17.5=7 torr=7760 atm=0.0092atmNow PV=nRT∴n=PVRT=0.0092×10.082×293=0.000382 mole=3.82×10−4mole