Question

- Air at a temperature of 20 0C and 750 mm Hg has a relative humidity of 80%. Calculate,

(i)The molal humidity of the air

(ii)The molal humidity of this air if its temperature is reduced to 10 °C and pressure increased to 2000 mm Hg condensing out some of the water

(iii)Weight of water condensed from 1000 litre of the original wet air

Vapour pressure of water at 20 °C = 17.5 mm Hg

Vapour pressure of water at 10 °C = 9.2 mm Hg

Answer 1

(1) Wolal 14umidity of me air Relative humidity Ye = 80% =0.8 Molat pressure of Ain Vapour pressure of = Relative water At 201 vip of water = 17.5 mm Pa = 0.8 17.5 mm Of Hg Pa= 0.8x17.5 = 14 mm Hg = 0.019 Molal huensidity & Pair P- pair P= Pressure of Ais = 750 mm Hg Molal Huenelity - 14 mwsity 650 — 145 mmity — 6) pressure increased to 2000 mm Hg. Temperature decreased to 10%. condensing implies due dew point Vapous prЛА - postnal pr&use Pa = Pva = 9.2 mm Hg molar hunnidity = Pair 9.2 - 750 P-P. (2000-9:2) - 5. 11x10 3

(iii) we have pu= MRT = PV R1. AL 20°c A P. 17.5 mm Hg = 14.5x0.001316 am = 0.02303 alms V = 1000L R = 0.082 halmolkamol T= 20 + 213) K 293K n= 0.02 30 zamx 1000L 0-0826 alm. I mol x 293 = 0.958 mole number of moles = mass moleulas mass = " mass = nx mm - 0.958 molx 18 g/mol = 17.2449