Modeling Tools for Environmental Engineers and Scientists

The given value of K a–w indicates that the gas phase content is quantifiedin partial pressure (atm.), and the aqueous phase content is quantified bymole fraction (mole/mole). Oxygen content in the atmosphere = 21% =mole fraction of 0.21. Because the atmospheric pressure is 1 atm., usingDalton’s Law, the partial pressure of oxygen in air = 0.21 × 1 atm.= 0.21 atm.0.21 atm.∴ Oxygen content in water = 40,000 atm .-molesHm 2OolesO= 5.25 × molesO10–6 2 m olesH O g2= 5.25 × 10 –6 molesO2m olesH O32g O2mg olesmoleO 1000 g m 18g H2O0gH O 100L = 9.3 m gL(2) In the given K a–w value, the gas phase content is in partial pressure form,and the aqueous phase content is in the mole fraction form. To convertthis value to the mole concentration ratio form, the gas phase content hasto first be converted from the partial pressure form to the molar concentrationform (moles/L). This can be achieved using the Ideal Gas Law:2222or,pV nRTn p R V TAssuming ambient temperature of T = 25ºC, and R = 82 atm.-L/kmole-K, at the partial pressure = 0.21 atm.,n 0.21 atm. V atm.-L82 k(273 + 25)Kmole-K= 8.6 × 10–6 km oleL = 8.6 × 10 –3 m oleLThe mole fraction in the aqueous phase was found as 5.25 × 10 –6 in part(I), which can be converted to molar concentration C using Equation(4.15):C A = X × C = (5.25 × 10 –6 ) × 55.5 gm oleL = 2.9 gm oleL© 2002 by CRC Press LLC