At a temperature of `20^(@)C` , vapour pressure of water is 17.5 torr. If relative humidity is 45% , calculate the moles of water present in one litre of air at `20^(@)C`.

Explanation

Relative humidity `= (" Partial pressure of " H_(2) O " in air ")/("Vapour pressure of " H_(2) O)`
` :.` Partial pressure of `H_(2) O ` in air , `p_(H_(2)O) = 0.45 xx 17.5 `
= 7.88 torr
= 0.0104 atm
PV = nRT
` n = ( PV)/( R T ) = ( 0.0 104 xx 1 )/( 0.0 821 xx 293 ) = 4.3 xx 10^(-4) mol`