Specific Gas Constant Calculator

What is the Specific Gas Constant?

The specific gas constant ($R$) is a materials property used in thermodynamics and fluid mechanics to represent the gas constant of a specific individual gas or mixture. Unlike the universal gas constant ($R^*$), which applies to all ideal gases on a per-mole basis, the specific gas constant works on a per-mass basis (e.g., per kilogram). It is crucial for calculations involving the ideal gas law in its density form: $P = \rho R T$.

Specific Gas Constant Formula

The specific gas constant is calculated by dividing the universal gas constant by the molecular weight (molar mass) of the gas:

$$R = \frac{R^*}{MW}$$

Where:

• $R$ = Specific gas constant of the gas, typically measured in $\text{J/(kg}\cdot\text{K)}$
• $R^*$ = Universal gas constant $\approx 8314.4626 \text{ J/(kmol}\cdot\text{K)}$ (or $8.3144626 \text{ J/(mol}\cdot\text{K)}$)
• $MW$ = Molecular weight (molar mass) of the gas, typically measured in $\text{kg/kmol}$ (or $\text{g/mol}$)

To find the molecular weight from a known specific gas constant, the formula can be rearranged:

$$MW = \frac{R^*}{R}$$

Specific Gas Constants of Common Gases

Because the specific gas constant is inversely proportional to molecular weight, lighter gases (like Hydrogen or Helium) have very high specific gas constants, whereas heavier gases (like Carbon Dioxide or Propane) have much lower values.