Question

Could I get an explanation of Henry’s Law including definition, examples, and how it was derivied if possible. I don't want the Wiki link, but I want to understand this law better.

Answer 1

Henry's Law states that the amount of a gas that dissolves in liquid is directly proportional to the partial pressure of that gas.

Formula and Henry constant

Henry's law may be mathematically expressed as:

• p=kH⋅c
• where:
• p is the partial pressure of the solute above the solution
• c is the concentration of the solute in the solution (in one of its many units)
• kH is the Henry's Law constant, which has units such as L··atm/mol or atm/(mole fraction) or Pa · m3/mol

Some values for kH include:

• oxygen (O2): 769.2 L · atm/mol
• carbon dioxide (CO2): 29.4 L · atm/mol
• hydrogen (H2): 1282.1 L · atm/mol

when these gases are dissolved in water at a temperature of 298 K.

Example Question 1: What is the concentration of O₂ in a fresh water stream in equilibrium with air at 25^oC and 1.0 bar? Express the answer in grams of O₂ per Kg of solvent.

Solution:
The mole fraction of O₂ in air is 0.21 and assuming the total pressure is 1.0bar, the partial pressure of O₂ is 0.21bar. Using this pressure for P_g in Henry's law we have

Solubility of O₂ = K_HP_g = $\frac{1.3 \times 10^{-3}mol}{Kg.bar}$ (0.21 bar) = 2.7 $\times$ 10^-4 mol/l

This concentration in grams per liter can then be calculated using the molar mass of O₂.

Solubility of O₂ = $\frac{2.7 \times 10^{-4}mol}{Kg}$ $\times$ $\frac{32.0g}{mol}$ = 0.0087 g/Kg

This concentration of O₂ (8.7 mg/Kg) is quite low, but it is sufficient to provide the oxygen required by aquatic life.