Question

a) The Henry’s constant for dichloromethane (DCM) is approximately 0.50 mol/(L atm) at 20°C. What is the dimensionless Henry’s constant? If the atmospheric concentration of DCM is measured to be 1.0 μg/m3, what is the concentration of DCM (in μg/L) in rainwater at equilibrium?

b) At 25°C, the Henry’s constant for carbon tetrachloride (CCl4) is 1.1. Assume an indoor pool in a poorly ventilated building has been filled with water contaminated with CCl4. If after reaching equilibrium with air at 25°C, the pool water concentration of CCl4 is 5.0 ppm, what will the gaseous concentration of CCl4 in the air be? Will it be in violation of the OSHA air standard which specifies a maximum of 10 ppmv? Show all work to defend your answer.

Answer 1

a) The dimensionless Henry's constant = Henry's constant x RT

= 0.50 mol-L-¹atm^-1 x 8.2 x 10^-2 L-atm-K^-1mol^-1 x 293 K = 12.01

The dimensionless Henry's constant can also be expressed as the dimensionless ratio between the aqueous-phase concentration of a species and its gas-phase concentration :

or, dimensionless Henry's constant = Ca/ Cg

therefore, Ca = Cg x dimensionless Henry's constant = 1.0 μg/m³ x 12.01 = 12.01 μg/m³ = 12.01 μg/ 1000 L

= 12.01 x 10^-3 μg/L.

B) dimensionless Henry's constant = Ca/ Cg

therefore, Cg = Ca/ dimensionless Henry's constant = 5.0 ppm / 1.1 = 4.55 ppm.