Henry\'s law may be expressed in different ways and with different concentration

Question

Henry's law may be expressed in different ways and with different concentration units, resulting in different values for the Henry's law constants. If mole fraction is used as the concentration unit, one algebraic statement of the law is: P_gas = k_H X_gas where k is the Henry's law constant in units of pressure, usually atm. At 25 degree C, some water is added to a sample of gaseous oxygen (O_2) at 4.02 atm pressure in a closed vessel and the vessel is shaken until as much oxygen as possible dissolves. Then 7.25 kg of the solution is removed and boiled to expel the oxygen yielding a volume of 1.02 L of O_2 (g) at 0 degree C and 1.00 atm. Determine the Henry's law constant for oxygen in water based on this experiment.

Explanation / Answer

moles of oxygen in the solution = PV/RT = 1* 1.02/(0.0821* 273)= 0.0455 moles

mass of oxygen =32*0.0455 =1.456 gm

Mass of water in the sample = 7.25*1000- 1.456 gm =7248.5 gm

moles of water = 7248.5/18= 403 moles

mole fraction of oxygen = 0.0455/0.0455+403)= 0.000113

KH= Pgas/mole fraction of oxygen = 4.2/0.000113= 37204 atm/mole fraction

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