Question

Consider a sealed container at 1 atm total pressure and 20 degrees C and containing both liquid and gas including air and a solution of 10 ppm chloroform in water. Use a water vapor pressure of 17.5 mmHg and a dimensionless concentration Henry's constant for chloroform of 0.120.

(a) What will be the concentration of chloroform in the gas phase? Express the answer in both ppmv and mg/m3.
Ans. _______________ ppmv

_______________ mg/m3

(b) A vacuum pump is connected to the container gas phase and operated for a period of time, then stopped and the container allowed to come to a new equilibrium. Essentially all of the air in the container is removed, but water vapor and chloroform will evaporate until equilibrium is reached. Assuming the liquid volume is very large so that the evaporation does not significantly change its concentration, what will be the new gas phase concentration, again in ppmv and mg/cu m, and what will be the total pressure?

Ans. ____________ ppmv

____________ mg/m3

Ptotal = ____________ atm.

Answer 1

As per Henry's Law, concentration of a solute gas in a solution is directly proportional to the partial pressure of the gas over the solution.

P = K_H C where

      P is the partial pressure of the gas above the solution; K_H is the Henry's Law constant for the solution & C is the concentration of the dissolved gas in solution.

C = P/K_H

Henry's Law can also be written as

K_H = C_aq / C_gas

Where, L_soln is liters of solution, c_aq is the concentration of gas, moles of gas per liter of solution and c_gas is concentration of component in gas phase

Henry's constant for chloroform, K_H = 0.120

C aq=10 ppm

K_H = C_aq / C_gas

C gas =10.0/0.12

        =83.3ppmv

Mass per m³ =       (83.3 x 10^-9 mols/1mol air) x (119.5g/mol) x (10³ mg/g)

                  =       0.00995 mg/mol air

Volume of one mole of air at P=1 atm and 293K is V=nRT/P

= (1mol) (0.08206 Lit atm/mol K)( 293K) / 1atm

= 24.09L

=0.02409 m³

Therefore concentration of chloroform in gas phase in mg/m³

= 0.00995/0.02409

=0.41 mg/m³