Calculate the specific gas constant (R_specific) for a gas based on its molar mass.
Last updated: March 2026 | By ForgeCalc Engineering
The specific gas constant (R_specific) is a property of a particular gas that relates its pressure, temperature, and volume. Unlike the universal gas constant (R_universal), which is the same for all ideal gases, the specific gas constant depends on the molar mass of the gas.
It is widely used in thermodynamics and fluid mechanics, especially in the Ideal Gas Law (P = ρRT) when working with mass density (ρ) instead of moles (n).
Where:
• R_specific is the specific gas constant (J/(kg·K))
• R_universal is approximately 8.31446 J/(mol·K)
• M is the molar mass of the gas (kg/mol)
For dry air, the specific gas constant is approximately 287.058 J/(kg·K). This is a fundamental constant in meteorology and aerodynamics.
The universal gas constant is defined per mole. Since different gases have different masses per mole, the constant must be adjusted to work with mass (kg) instead of molar quantity.
Water vapor has a lower molar mass (18.02 g/mol) than dry air (28.97 g/mol). Therefore, moist air has a higher specific gas constant than dry air.
The law is written as P × V = m × R_specific × T, where m is the mass of the gas. Alternatively, P = ρ × R_specific × T, where ρ is the density.
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