Why 'R 'is The Universal Gas Constant? What Does It Mean? - LinkedIn

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There are two constants that engineers use in some form almost every day in calculations [1] Gas constant and [2] Universal constant. While a gas constant is specific for gas the universal gas constant applies to all gases.

The question is why? What is so special in universal gas constant ‘R ‘that makes it apply to all gases?

In fact, if you go deep, the fundamental equation of state PV = n RT which combines Charles, Boyle, and Gay-Lussac gas law made R evolve as a universal constant.

There is a misconception that ‘ R’ is a universal gas constant for only ideal gases and it applies to only PV = n RT. The truth is it is not. It applies to all gases, let us look at the Van der Walls equation for real gases, [P + a(n/V)2] (V – nb) = nRT. Van der walls equation uses the same R as the ideal gas law equation.

Then what is ‘ R ‘?

The gas constant R is defined as the Avogadro constant NA multiplied by the Boltzmann constant k (or kB):

R=NA x k Joule / mole / k

Avogadro’s number: Its SI unit is the reciprocal mole, and it is defined exactly as NA = 6.02214 ×10²³ mol⁻¹. For 1 mole this is constant.

Boltzmann constant: The Boltzmann constant (kB or k) is the proportionality factor that relates the average relative kinetic energy of particles in a gas with the thermodynamic temperature of the gas. At a given T this k is constant. The value of k = 1.38065 × 10^23 J⋅K^-1

R relates the average kinetic energy of particles in one-mole gas with temperature. At constant temp R simply relates to the kinetic energy of particles in a gas.

As a consequence, the SI value of molar gas constant R=NA x k is exactly 8.31432 J⋅K−1⋅mol−1.

Since ‘R ‘is a product of two universal constants ‘R’ is a universal constant independent of any particular gas

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