Boyle’s, Charles & Gay-Lussac’s Law from Ideal Gas Law

The Ideal Gas Law and Avogadro’s Number

In terms of Avogadro’s number NA, we can write the number of moles n as:

1) n = N/NA

where N represents the total number of molecules in a gas and NA = 6.02 x 1023 molecules/mole. We found in part one that the ideal gas law may be written as:

2) PV = nRT

Substituting 1 into 2 yields:

3) PV = N/NART

We can simplify this equation even further with the use of Boltzmann’s contant. Boltzmann’s constant is defined as:

4) k = R/NA

which is 1.38 x 10-23 J/K in SI units. Then equation 3 becomes:

5) PV = NkT

This is another standard way of writing the ideal gas equation.

Another Form

It is not always necessary to use the number of molecules or Boltzmann’s constant.

Ideal Gas Law

In this case, the ideal gas law is also commonly written as:

6) P1V1 / T1 = P2V2 / T2

where P1,V1, and T1 are the original values of the gas, while P2,V2, and T2 represent its final values.

We can use equation 6 to derive Boyle’s, Charles’, and Gay-Lussac’s Laws. We do this by considering isothermal, isobaric, and isochoric thermodynamic processes.

Derivation of Boyle’s Law


Isothermal means that the temperature is constant. When we do this, T1 = T2 = T, so:

7) P1V1 / T = P2V2 / T

The Ts cancel, and we are left with Boyle’s Law P1V1 = P2V2.

Derivation of Charles’ Law


Isobaric means that the pressure is constant, so P1 = P2 = P, giving us:

8) PV1 / T1 = PV2 / T2

With the Ps canceling, we are left with Charles’ Law V1/T1 = V2/T2.

Derivation of Gay-Lussac’s Law


Finally, isochoric means the volume is constant, such that V1 = V2 = V, and thus we have:

9) P1V / T1 = P2V / T2

The Vs cancel, giving us Gay-Lussac’s Law P1/T1 = P2/T2.


Physics for Scientists and Engineers by Douglas Giancoli

Fundamentals of Physics by Halliday, Resnick, and Walker

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Ideal gas law from


Comprehensive List of the Various Forms of the Ideal Gas Law

This post is part of the series: Introduction to the Ideal Gas Law

This series gives an elementary, non-calculus based introduction to the ideal gas law, including an account of its origins and how to use it to derive Boyle’s, Charles’, and Gay-Lussac’s Laws.
  1. What is The Ideal Gas Law?
  2. The Ideal Gas Law and the Mass of Air Inside Your Bedroom
  3. Other Forms of the Ideal Gas Law