# Gas laws in chemistry

### Gas Laws:

Gases are considered as one of the states of matter. They generally show similar behaviour when certain conditions like pressure, temperature, volume, mass and amount of substance (mole) are equal. However, a slight change in one of these conditions will result into a deviation. These deviations are analyzed and calculated through the gas laws.

The gas laws were created around the 18th century by some notable scientists. As stated earlier, these laws basically describe the behaviour of gases at varying conditions like temperature, pressure and volume. The laws are named after the scientists who discovered them. They include the following:

1. Boyle's Law: This law was described by an English scientist named Robert Boyle in 1662. He worked on the relationship between the pressure and volume of a gas.

Boyle's law states that the volume of a gas is inversely proportional to its pressure provided the temperature remains constant.

This law is also referred to as Mariotte's law.

• PV = k

• P = Pressure

• V = Volume

• k = Constant

2. Charles's Law: This law is sometimes referred to the law of volumes. It was formulated by a French physicist named Jacques Charles in 1780. He worked on the relationship between the volume occupied by a gas and its absolute temperature.

Charles's law state that the volume of a given mass of a gas is directly proportional to its absolute temperature, provided the pressure remains constant.

• V = T

• V/T = k

• V = volume

• T = Temperature

• k = Constant

3. Avogadro's Law: This law (or hypothesis) is named after Amedeo Avogadro (August 9, 1776 - July 9, 1856), an Italian mathematical physicist. He formulated his hypothesis in 1812. He worked on the relationship between the volume occupied by a gas and the amount of gaseous substance.

Avogadro's law states that equal volumes of all gases, at the same temperature and pressure, contain the same number of molecules.

• V = n

• V/n = k

• V = Volume of gas

• n = amount of substance for the gas (measured in moles)

• k = Constant

4. Gay Lussac's Law: This law is sometimes referred to as the law of combining volumes of gas; and was put forward by a French scientist, Joseph-Louis Gay-Lussac's law in 1808.

Gay Lussac's law of combining volumes states that, when gases undergo chemical reaction, they do so in simple whole number ratios to one another, provided that the temperature and pressure of the reacting gases and their products remain constant.

According to the law of combining volumes, the ratio between the volumes of gaseous reactant and products are expressed in simple whole numbers.

Meanwhile, understand that Gay-Lussac's law also shows a relationship between the pressure and temperature of a gas at constant volume. It states thus:

• At a constant mass and volume, the pressure of a gas is directly proportional to the temperature. Therefore, as temperature increases, the pressure will also increase and vice versa. This law may sometimes be referred to as Amonton’s law.

The proportionate increase in pressure and temperature results from the collision of gas molecules with one another and the walls of the container.

• P = T

• P/T = k

• P = Pressure exerted by the gas

• T = Absolute temperature of the gas

• k = Constant.

5. Dalton's Law of Partial Pressures: This law may simply be referred to as Dalton's law, and it is applicable to ideal gases. It is an empirical law put forward by John Dalton, an English chemist, in 1801. This law determines the individual pressures of each gas in a mixture of gases. By mixture of gases, we mean that the gases are only physically combined, and not chemically combined.

Dalton's law of partial pressures states that the total pressure of gases exerted in a mixture of non-reacting gases is equal to the sum of the partial pressures of each of the component gases.

The pressure exerted by each gas is termed partial pressure.

• PTotal = PGas 1 + PGas 2 + PGas 3 + ... PGas n

• Where PTotal = Total pressure of the gaseous mixture.

• PGas = Partial pressure of each gas

You can read on changes in temperature and pressure in dynamic equilibrium here.

If the total pressure is known and the moles of each component gas are known, the partial pressure can be calculated using the formula:

• Px = PTotal (nx / nTotal)

• Px = Partial pressure of gas x

• PTotal = Total pressure of all gases

• nx = Number of moles of gas x

• nTotal = Number of moles of all gases

If the total gas is collected over water, then it may become saturated with water vapour. In this instance, the total pressure becomes:

• PTotal = PGas + PWater vapour

You can read on physical and chemical change in chemistry here.

### General Gas Equation

The gas laws; that is: Boyle's law, Charles's law, Avogadro's law, Gay Lussac's law and Dalton's law of partial pressures have opened up our understanding on the relationship between the temperature, pressure, volume and amount of a sample of gas. Together, these gas laws are combined into a general gas equation or the ideal gas law.

The ideal gas law states that the volume of a given amount of a gas is directly proportional to the number of moles of the gas, directly proportional to its kelvin temperature and inversely proportional to the pressure.

pV = nRT

This law was first stated by Benoît Paul Émile Clapeyron in 1834.

In order to derive the general gas equation (pV = nRT), three laws were conbined. These are: Boyle’s law, Charles’s law and Avogadro’s law respectively. This is summarized below:

• Boyle’s law V ∝ 1/P (where T and n are constant)

• Charles’s law V ∝ T (where P and n are constant)

• Avogadro’s law V ∝ n (where P and T are constant)

• V = Volume, P = Pressure, T = Temperature and n = Amount of gas in moles.

If we combine these laws, we will have:

V ∝ nT/P

pV= nRT

Where R is a proportionality constant

This is the ideal gas equation. This equation is applied in solving problems with a change in any of these four variables, that is: n, P, V and T.

You can read on molecular mass here.

It is noteworthy to state that the ideal gas law summarizes the behavior of gases under various conditions; an can be used to solve problems regarding a change in the n, P, V and T variables. However, it still has its limitations. 