Calculate the mass of water vapour present in a room of volume 400 m3 that contains air at 27 °C on a day when the relative humidity is 60 per cent. Hint: Relative humidity is the prevailing partial pressure of water vapour expressed as a percentage of the vapour pressure of water vapour at the same temperature (in this case, 35.6 mbar).

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Answer:

Mass of water = 6251. 706g or 6.25Kg

Explanation:

Relative humidity = (actual vapor pressure/saturation vapor pressure) * 100%

Actual vapor pressure, Pw = relative humidity * saturation vapor pressure

Pw = 60% * (35.6 *0.001)atm = 0.0216atm

Note: 1mbar = 0.001atm

Using the ideal gas equation: PV=nRT; where P = Pw= 0.02136atm, V= (400

* 1000)dm^3, R= 0.082 atmdm^3/kmol, T= (27+273)K, n = number of moles

Note: 1m^3 = 1000dm^3,R is the molar gas constant.

Making n subject of the formula, n = PV/RT

n= (0.02136 * 400000)/(0.082 * 300) = 347.317 moles

Mass (g) = number of moles (n) * molar mass

molar mass of water=18g

Mass of water = 347.317 * 18 = 6251. 706g or 6.25Kg

The relative humidity is given as the ratio of the actual vapor pressure and the saturation vapor pressure. The mass of the water vapor is 6.25 kg.

What is humidity?

The humidity is given as the mass of the water vapor present in the air.

The vapor pressure of the water in the air is calculated as:

[tex]\rm Vapor\;pressure=Realtive\;humidity\;\times\;saturation\;vapor\;oressure\\Vapor\;pressure=60\;\times\;(35.6\;mbar\;\times0.001)atm\\Vapor\;pressure=0.0216\;atm[/tex]

The moles of the water has been calculated by the ideal gas equation as:

[tex]\rm PV=nRT\\0.0216\;\times\;400000\;dm^3=n\;\times\;0.0821\;atm.dm^3/mol.T\;\times\;300\;K\\n=347.317 \;mol[/tex]

The molar mass of water is 18 g/mol, the mass of water vapor in 347.317 moles is:

[tex]\rm Mass=Molar\;mass\;\times\;Moles\\Mass=18\;g./mol\;\times\;347.317\;mol\\Mass=6251.706\;g\\Mass=6.25\;kg[/tex]

The mass of water vapor in the air is 6.25 kg.

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