The Ideal Gas Equation

The three individual gas laws — Boyle's, Charles's, and the pressure law — each describe what happens when one variable changes while the others are held constant. The ideal gas equation combines all three into a single, powerful equation that describes the behaviour of an ideal gas under any conditions.

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The Molar Form: pV = nRT

The most commonly used form of the ideal gas equation is:

pV = nRT

where:

• p = pressure (Pa)
• V = volume (m³)
• n = number of moles of gas (mol)
• R = molar gas constant = 8.31 J mol⁻¹ K⁻¹
• T = absolute temperature (K)

This equation tells you that for a given amount of gas, the product pV is proportional to the absolute temperature. If you know any three of the four variables (p, V, n, T), you can calculate the fourth.

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The Molecular Form: pV = NkT

An alternative form uses the total number of molecules rather than moles:

pV = NkT

where:

• N = total number of molecules
• k = Boltzmann constant = 1.38 × 10⁻²³ J K⁻¹