Henry’s Law Explained: Definition, Formula, Examples & Applications for JEE and NEET

 

Introduction

In chemistry, the solubility of gases in liquids is an important concept that helps explain many natural and industrial processes. One of the fundamental principles describing this behavior is Henry’s Law. This law explains how the pressure of a gas affects the amount of gas that dissolves in a liquid.

Henry’s Law, proposed by William Henry in 1803, is widely used in chemistry, environmental science, and engineering.

What is Henry’s Law?

Henry’s Law states that:

At constant temperature, the amount of gas dissolved in a liquid is directly proportional to the pressure of that gas above the liquid.

In simple words:

• Higher pressure → More gas dissolves in the liquid

• Lower pressure → Gas escapes from the liquid

Mathematical Expression of Henry’s Law

The formula of Henry’s Law is:

$$C = k_H \times P$$

Where:

• C = concentration of dissolved gas in the liquid

• kH = Henry’s Law constant

• P = partial pressure of the gas

The value of kH varies depending on the type of gas, liquid, and temperature.

Everyday Examples of Henry’s Law

1. Carbonated Beverages

Soft drinks contain carbon dioxide dissolved under high pressure. When the bottle is opened, the pressure decreases and carbon dioxide escapes as bubbles.

2. Scuba Diving

During deep sea diving, high pressure causes more nitrogen to dissolve in the diver’s blood. If the diver ascends quickly, nitrogen bubbles may form in the bloodstream causing Decompression sickness.

3. Aquatic Life

Fish and other aquatic organisms depend on dissolved oxygen in water. The amount of oxygen dissolved depends on atmospheric pressure and temperature.

Factors Affecting Henry’s Law

1. Pressure

An increase in pressure increases the solubility of gases in liquids.

2. Temperature

Increasing temperature generally decreases the solubility of gases in liquids.

3. Nature of Gas and Liquid

Different gases have different solubilities depending on their chemical properties.

Applications of Henry’s Law

Henry’s Law has many practical uses:

• Production of carbonated beverages

• Designing breathing equipment for divers

• Environmental studies of gas exchange in oceans

• Chemical engineering processes involving gases

Factors Affecting Henry’s Law Constant (kH)

The Henry’s Law constant (kH) determines how easily a gas dissolves in a liquid. A higher value of kH generally indicates lower solubility of the gas in the liquid, while a lower value indicates higher solubility.

Several factors influence the value of the Henry’s constant.

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1. Temperature

Temperature has a major effect on the value of the Henry’s constant.

• As temperature increases, the solubility of gases in liquids generally decreases.

• Therefore, the value of kH increases with increase in temperature.

Reason:
Dissolving a gas in a liquid is usually an exothermic process. Increasing temperature supplies energy to gas molecules, allowing them to escape from the liquid.

Example:
Warm soda loses carbon dioxide faster than cold soda.

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2. Nature of the Gas

Different gases have different interactions with liquids.

• Gases that interact strongly with the solvent dissolve more easily.

• Weakly interacting gases dissolve less.

Example:

• Ammonia (NH₃) dissolves easily in water due to hydrogen bonding.

• Oxygen (O₂) and Nitrogen (N₂) dissolve less because they interact weakly with water molecules.

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3. Nature of the Solvent

The solvent also plays an important role.

• Polar solvents dissolve polar or reactive gases better.

• Non-polar solvents dissolve non-polar gases better.

Example:

• Carbon dioxide dissolves well in water.

• Non-polar gases dissolve better in organic solvents than in water.

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4. Pressure of the Gas

Although the Henry’s constant itself is independent of pressure at constant temperature, the amount of gas dissolved depends on pressure according to Henry’s Law:

$$C = k_H P$$

When pressure increases, more gas dissolves in the liquid.

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5. Presence of Other Solutes

If other substances (such as salts) are dissolved in the liquid, the solubility of gases may decrease.

This effect is known as the salting-out effect.

Example:
Oxygen dissolves less in saltwater compared to pure water.

Numericals Based on Henry’s Law

The Henry’s law constant for a gas in water is 1.2 × 10⁻³ mol/L·atm.
If the pressure of the gas is 3 atm, calculate the concentration of the dissolved gas.

Solution

Formula:

$$C = k_H \times P$$

Given:

kH = 1.2 × 10⁻³ mol/L·atm
P = 3 atm

Calculation:

$$C = (1.2 × 10^{-3}) × 3$$ $$C = 3.6 × 10^{-3} \, mol/L$$

Answer

The concentration of dissolved gas = 3.6 × 10⁻³ mol/L

If the concentration of a dissolved gas is 0.02 mol/L and Henry’s constant is 0.01 mol/L·atm, calculate the pressure of the gas.

Solution

Formula:

$$P = C / k_H$$

Given:

C = 0.02 mol/L
kH = 0.01 mol/L·atm

Calculation:

$$P = 0.02 / 0.01$$ $$P = 2 atm$$

The solubility of a gas in water is 0.04 mol/L at 4 atm pressure.
Find the solubility at 8 atm pressure if temperature remains constant.

From Henry’s Law:

$$C_1 / P_1 = C_2 / P_2$$

Given:

C₁ = 0.04 mol/L
P₁ = 4 atm
P₂ = 8 atm

Calculation:

$$C_2 = (C_1 × P_2) / P_1$$ $$C_2 = (0.04 × 8) / 4$$ $$C_2 = 0.08 mol/L$$

Answer

The solubility of the gas at 8 atm pressure is 0.08 mol/L.

Conclusion

Henry’s Law is a fundamental concept in chemistry that explains how gases dissolve in liquids depending on pressure. It has many practical applications in everyday life and scientific fields such as environmental science, chemical engineering, and diving physiology.

1.

The Henry’s law constant for a gas in water is 1.5 × 10⁻³ mol L⁻¹ atm⁻¹.
Calculate the concentration of the gas dissolved in water when the pressure of the gas is 4 atm.

2.

The concentration of a gas dissolved in water is 0.025 mol/L.
If Henry’s constant is 0.005 mol L⁻¹ atm⁻¹, calculate the pressure of the gas.

3.

The solubility of a gas in water is 0.03 mol/L at 2 atm pressure.
Find the solubility at 5 atm pressure, assuming temperature remains constant.

4.

A gas has a solubility of 0.04 mol/L in water at 2 atm pressure.
Calculate the value of Henry’s constant

5.

The solubility of oxygen in water is 0.0013 mol/L at 1 atm pressure.
Determine the solubility of oxygen at 3 atm pressure.

Thank you !!!

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