Question

2. The molarity (moles of oxygen per liter of solution) based Henry’s law constant for oxygen in freshwater is 620.3 (liter atm)/mol at 288 K. What is the solubility (in mole fraction) of oxygen in freshwater at ambient pressure and composition of air? Your number should be a good estimate of the oxygen composition that fish need to survive. If the oxygen composition drops by just 10% of the value you calculate (which occurs at a bit over 30°C) then most fish species start having trouble. Hope no one has overheated their fish tank and killed off the fish because the oxygen level got too low.

Answer 1

Henry's Law for Solubility of Dissolved gases in liquid relates the gas solubility in the liquid and its partial pressure through a constant called Henry's Law constant. This law is only applicable to gases if they are present in low concentrations(dilution, as is the case with the dissolution of oxygen and carbon dioxide in liquid water) in the liquid. For very high concentrations of dissolved gases in the liquid, Lewis Randall's Rule is applied for calculation of the concentrations.

Henry's Law is given by:

where for species A ( here oxygen):

= Solubility of A in the liquid

= Henry's Law constant

= PArtial Pressure of A over the liquid

The partial pressure of oxygen over liquid water at ambient conditions is 0.2 atm.

Usually fishes need 5-6 ppm of dissolved oxygen for growth and better metabolism.