Henry’s law
- Henry’s Law establishes aquantitative relation betweenpressure and solubility of a gasin a solvent.
- This law is for gas-liquid solution.
- According to the law at a constant temperature, thesolubility of a gas in a liquid is directly proportional to thepressure of the gas.
- It can also be stated as - The partial pressure of the gas in vapour phase (p) is proportional to the mole fraction of the gas (x) in the solution.
- Mathematically,
p = KH x where KH = Henry’s law constant.
- At same temperature different gasses have different KH
- It is a function of the nature of the gas. At a given pressure increasing value of KH implies lower solubility of the gas in the liquid.
- Value of KH increases with the increase in temperature therefore solubility of gases increases with decreasing temperature. Due to this reason cold water is more sustainable for aquatic life than warm water.
Problem: H2S, a toxic gas with rotten egg like smell, is used for the qualitative analysis. If the solubility of H2S in water at STP is 0.195 m, calculate Henry's law constant.
Solution:
It is given that the solubility of H2S in water at STP is 0.195 m, i.e., 0.195 mol of H2S is dissolved in 1000 g of water.
Moles of water = 1000g / 18g mol-1 = 55.56 mol
∴Mole fraction of H2S, x = Moles of H2S / Moles of H2S+Moles of water 0.195 / (0.195+55.56)= 0.0035
At STP, pressure (p) = 0.987 bar
According to Henry's law:
p = KH x ⇒ KH = p / x
= 0.0987 / 0.0035 bar
= 282 bar
