Solubility and Henry's Law - Practice Questions & MCQ

Henry’s Law:

Henry was the first to give a quantitative relation between pressure and solubility of a gas in a solvent. The law states that at a constant temperature, the solubility of a gas in a liquid is directly proportional to the partial pressure of the gas present above the surface of liquid or solution. If the solubility of gas be expressed in terms of its mole fraction in the solution, then it can be said that the mole fraction of gas in the solution is proportional to the partial pressure of the gas over the solution. Alternatively, the most commonly used form of Henry’s law states that “the partial pressure of the gas in vapour phase (P) is proportional to the mole fraction of the gas (x) in the solution” and is expressed as:

Here KH is the Henry’s law constant and has the same units as the units of pressure used in the equation. It can be clearly seen that the plot between partial pressure of the gas versus mole fraction of the gas in solution will be a straight line plot as shown in figure given below.

Factors governing the value of K_H:

Different gases have different KH values at the same temperature. Also, the same gas has different values of KH at different temperatures which is shown in the table given below. The factors governing the value of Henry’s constant is given below

1. Nature of gas-solvent interaction

As gas-solvent interactions become stronger, the solubility will increase keeping the pressure constant and thus the value of KH will decrease. For example, HCl has a lower value of Henry’s constant as compared to O2

2. Temperature: 

As the temperature increases, the solubility of the gas decreases and hence keeping the pressure constant for the same gas, the value of KH will increase.

Applications of Henry’s law: 

(1) Soda bottle fizzes when opened: When the soda bottle is opened, then the pressure decreases in the bottle. Now, due to this decrease in pressure, the solubility of gas decreases. Now if we leave this bottle opened for some time, then all fizz goes out and we do not feel the drinking.

(2) Anoxia at higher altitudes: This is the condition of tiredness and mental confusion at higher altitudes. At higher altitudes, pressure decreases and thus the solubility of oxygen gas in the body decreases which causes anoxia.

(3) Avoiding Bends in Scuba divers: When scuba divers go deep into the ocean, then pressure increases. Now due to this increase in pressure the solubility of gases in the blood increases. Further, when these divers come up at the sea level, then pressure decreases, and the solubility of gases in the blood decreases. Due to this decrease in solubility, the gases come out of the capillaries in the form of bubbles which causes a serious medical condition called as bends. To avoid bends, Helium is used in the diving tanks