Henry’s law is important in environmental chemistry, where it predicts the distr

Question

Henry’s law is important in environmental chemistry, where it predicts the distribution of pollutants between water and the atmosphere. The hydrocarbon cyclopropane (C3H6) emitted in wastewater streams, for example, can pass into the air, where it is degraded by processes induced by light from the sun. The Henry’s law constant for cyclopropane in water at 25 °C is 4273 atm, when the following form of the law is used: Pcyclopropane = kcyclopropane Xcyclopropane Calculate the partial pressure of cyclopropane vapor in equilibrium with a solution of 1.84 g of cyclopropane per 1010 L of water. How many cyclopropane molecules are present in each cubic centimeter of vapor?

Explanation / Answer

H = 4273 atm

P = H*X

find partial pressure of cuclopropane if

m = 1.84 g of gas is present in 1010 L of water

mol of gas = mass/MW = 1.84/42.08 = 0.0437262

mol of water = mass/MW = 1010/18 = 56.1111

x solute = 0.0437262/(56.1111+0.0437262) = 0.000778

so

Ppartial = H*x = 4273*0.000778 = 3.32439 atm

b)

find molecules per unit cm3

so

PV = nRT

n = PV/(RT)

n = (3.32439)(0.001)/(0.082*298) = 0.000136044 mol

1 mol = 6.022*10^23

so

molecules = (0.000136044)(6.022*10^23) = 8.1925*10^19 molecules per 1 cm3

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