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How It Works
The specific gas constant (R) is the universal gas constant divided by a gas's molecular weight: R = R* / MW. It lets you work with mass (kg) instead of moles in thermodynamic equations like the density form of the ideal gas law, P = ρRT.
The universal gas constant R* = 8314 J/(kmol·K) applies to every ideal gas. Once you divide by molecular weight, R becomes specific to one gas — for example, 287 J/(kg·K) for dry air.
Example Problem
Find the specific gas constant for carbon dioxide (CO₂), which has a molecular weight of 44.01 kg/kmol.
- Apply the formula: R = R* / MW
- R = 8314 / 44.01
- R ≈ 188.9 J/(kg·K)
This means each kilogram of CO₂ at 300 K and 1 atm occupies significantly less volume than 1 kg of hydrogen (R ≈ 4124), because heavier molecules pack more mass into the same space.
Frequently Asked Questions
What is the specific gas constant for air?
Dry air has a molecular weight of about 28.97 kg/kmol, giving it a specific gas constant of approximately 287 J/(kg·K). This value is used throughout meteorology and aerospace engineering.
How is the specific gas constant different from the universal gas constant?
The universal gas constant R* = 8314 J/(kmol·K) works with moles and applies to any ideal gas. The specific gas constant R is R* divided by the molecular weight, so it works with mass and is unique to each gas.
Can you use the specific gas constant for gas mixtures?
Yes. For a gas mixture, calculate an effective molecular weight as a mole-fraction weighted average of each component's molecular weight, then divide R* by that effective value. Air itself is a mixture (mostly N₂ and O₂) treated this way.
What are the units of the specific gas constant?
The SI unit is J/(kg·K), which is equivalent to m²/(s²·K). Some references express it in kJ/(kg·K) for convenience. Always match units with the rest of your equation.
Related Calculators
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