Henry's Law: Definition, Formula, Applications and Limitations

What is Henry's Law?

Henry's law deals with the solubility of the gas in a liquid at a particular temperature. This law was given by Henry in 1803 which states that The mass of a gas dissolved in a given volume of the liquid at constant temperature is directly proportional to the pressure of the gas present in equilibrium with the liquid.

We can understand Henry's law by a simple experiment. Let us consider a dynamic equilibrium system shown in the given figure. The lower part of the system is liquid and its upper part is filled with a gas having pressure P and temperature T.

Henry's law demonstrating the effect of pressure on solubility of a gas in a liquid

Now, slowly increase the pressure over the system as shown in the figure. You will notice that the concentration of particles of gas in the liquid increases i.e gas is getting dissolved in the liquid.

The more you increase the pressure, the more gas will dissolve in the liquid till a saturation point is attained. This is nothing but Henry's law.


Mathematical Derivation of Henry's Law

Mathematically we can derive the formula for Henry's law as given below:

m ∝ Kp or m = Kp ----------------- (1)

Where m = mass of the gas dissolved in a unit volume of solvent.

p = pressure of the gas in equilibrium with the solvent.

K = constant whose value depends upon the nature of gas, solvent, and temperature.

Later it was also concluded by experiment that if a mixture of gases is simultaneously in equilibrium with the liquid at a particular temperature, the solubility of either gas in the mixture is directly proportional to the partial pressure of that gas in the mixture.

So, Henry law can be modified as

XA = K'pA ----------------- (2)

Where, XA = mole fraction of the gas in the solution.

pA = partial pressure of the gas.

K' = constant whose value depends upon the nature of gas, solvent, and temperature.

Equation No. 2 can be written as pA = 1/K'.XA or 

pA = Kh.XA

Where, Kh = 1/K' is called Henry's constant

This is the most common expression for Henry's law.


Applications of Henry's law

1. In the production of soft drinks

To increase the solubility of CO2 in soft drinks, the bottles are sealed under high pressure. When the seal of the bottle is broken, CO2 comes out and its solubility decreases in the soft drink.

2. In scuba diving

Scuba divers need oxygen to survive undersea. According to henry's law, under high-pressure solubility of gas increases. So, the solubility of oxygen and nitrogen increases under the sea. Oxygen is used up for metabolism activities but nitrogen remains as it is in blood. 

When scuba diver comes out of the sea, dissolved nitrogen starts releasing from the blood causing decompression sickness. So to minimize the effect of decompression sickness, little amount of helium is also added to the cylinder along with oxygen and nitrogen.

3. In the functioning of lungs

When air enters the lungs, the partial pressure of oxygen is high. This oxygen combines with hemoglobin to form oxyhemoglobin which is utilized for the function of the cells.

4. For mountain climbers

On high altitudes, due to low atmospheric pressure the solubility of oxygen decreases which causes different problems like weakness, unable to think properly. This is known as altitude sickness.


Limitations of Henry's law

Following are some limitation of Henry's law

1. The gas should behave as an ideal gas, then only it will follow Henry's law.

2. The gas should not form any compound with solvent i.e association or dissociation of gas molecules should not place with solvent.

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