A room contains 35 kg of dry air and 0.5 kg of water vapor.

A room contains 35 kg of dry air and 0.5 kg of water vapor. The total pressure and temperature of air in the room are 100 kPa and 25°C respectively. Given that the saturation pressure for water at 25°C, is 3.17 kPa, the relative humidity of the air in the room is

m_a = 35 kg, m_v = 0.5 kg

w =	m_v	=	0.5	= 0.01428
	m_v		35

also w = 0.622	P_v
	P_a

P_t = 100 kP_a

∴ w = 0.622 ×	P_v
	P_t - P_v

Now, 0.01428 = 0.622 ×			P_v
			P_t - P_v

P_v = 2.238 kPa
Relative humidity

φ =	P_v	=	2.238	× 100
	P_s		3.17

φ = 70.6% = 71%