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 methylacetylene (C3H4) 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 methylacetylene in water at 25 °C is 601 atm, when the following form of the law is used: Pmethylacetylene = kmethylacetylene Xmethylacetylene Calculate the partial pressure of methylacetylene vapor in equilibrium with a solution of 1.59 g of methylacetylene per 1220 L of water. How many methylacetylene molecules are present in each cubic centimeter of vapor?

Explanation / Answer

Pmethylacetylene = kmethylacetylene Xmethylacetylene

X = mole fractino of methyl acetylene, moles of methyl acetylene= mass/ molar mass = 1.59/40 =0.03975

mass of water= density* volume in ml, volume of water = 1 g/ml*1220L*1000ml= 1.220*106 gm

moles of water= mass/molar mass = 1.220*106/18 =67778

mole fraction of methyl acetylene= moles of methyl acetylene/ totl moles = 0.03975/(0.03975+67778)=5.86*10-7

partial pressure= 601* atm* 5.86*10-7=0.00035 atm

this is the pressure if it alone occupies the entire volume of gas.

from gas law , PV= nRT

n = no of moles , n/V =moles/volume in cm3, R = 0.0821 L.atm/mole.K= 0.0821*1000cc/mole.K =81.2 cc/mole.K

T= 25 deg.c= 25+273 =298K

n/V= P/RT =0.00035/(82.1*298) = 1.43*10-8 moles/cm3

1 mole contains 6.023*1023 molecules

hence no of molecules per cm3= 1.43*10-8*6.023*1023 molecues/cm3=8.62*1015 moles/cm3

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