# Universal Gas Constant vs. Characteristic Gas Constant — What's the Difference?

By Tayyaba Rehman — Published on January 30, 2024
The Universal Gas Constant is a constant for all ideal gases, used in the ideal gas law, while the Characteristic Gas Constant is specific to a particular gas, varying based on its molecular weight.

## Key Differences

The Universal Gas Constant (R) is a constant value used in the equation of state for an ideal gas, represented in various units, commonly J/(mol·K). In contrast, the Characteristic Gas Constant (R-specific), also known as the individual or specific gas constant, is unique to each gas and is derived by dividing the Universal Gas Constant by the molar mass of the gas.
R is crucial in the ideal gas law (PV=nRT), linking pressure, volume, temperature, and the amount of gas. Whereas R-specific is used in equations tailored for a specific gas, such as PV=mR-specificT, where m is the mass of the gas.
The Universal Gas Constant provides a consistent value that applies to all ideal gases, facilitating calculations in chemistry and physics. In contrast, R-specific, being dependent on the molecular weight of a gas, varies and thus provides more precise values for calculations involving a specific gas.
In practical applications, R is used for general gas law calculations, making it fundamental in various scientific fields. R-specific, however, is more often used in engineering and specialized scientific calculations where the properties of a specific gas are known.
The value of R is universally recognized and constant, while the value of R-specific changes depending on the gas in question, reflecting the intrinsic properties of different gases such as density and molecular weight.

## Comparison Chart

### Definition

Constant for all ideal gases
Specific to each individual gas

R
R-specific

### Usage

Used in the ideal gas law
Used in specific gas equations

### Dependence

Constant value (8.314 J/(mol·K))
Varies with the molecular weight of the gas

### Application

General gas law calculations
Calculations for a specific gas type

## Compare with Definitions

#### Universal Gas Constant

The Universal Gas Constant is fundamental in thermodynamics.
R is a staple constant in thermodynamic equations.

#### Characteristic Gas Constant

The Characteristic Gas Constant is specific to each gas, based on its molecular weight.
For nitrogen, I use its specific R value in my calculations.

#### Universal Gas Constant

The Universal Gas Constant is a constant in the ideal gas law.
In my chemistry class, we used R to calculate the pressure of a gas.

#### Characteristic Gas Constant

R-specific is used in specialized gas equations.
In engineering, we often use R-specific for precise gas system calculations.

#### Universal Gas Constant

The Universal Gas Constant has a value of 8.314 J/(mol·K).
When calculating the energy of a gas, I use R's value of 8.314 J/(mol·K).

#### Characteristic Gas Constant

The Characteristic Gas Constant reflects the unique properties of each gas.
The R-specific for carbon dioxide is different due to its unique density and molecular weight.

#### Universal Gas Constant

R is applicable to all ideal gases.
R remains constant regardless of the type of gas used in our experiments.

#### Characteristic Gas Constant

R-specific varies for different gases, unlike the universal R.
Calculating the energy for oxygen requires its unique Characteristic Gas Constant.

#### Universal Gas Constant

R is crucial for relating pressure, volume, and temperature in gases.
Understanding the Universal Gas Constant is key to mastering gas laws.

#### Characteristic Gas Constant

The Characteristic Gas Constant is derived by dividing R by the gas's molar mass.
To find the R-specific for helium, divide the Universal Gas Constant by helium's molar mass.

## Common Curiosities

#### What is the Universal Gas Constant?

The Universal Gas Constant is a constant value used in the ideal gas law for all ideal gases.

#### What's the value of the Universal Gas Constant?

It's approximately 8.314 J/(mol·K).

#### How is the Characteristic Gas Constant different?

It's specific to each gas, calculated based on the gas's molecular weight.

#### Can R-specific be used for any gas?

No, R-specific is unique to each gas and varies accordingly.

#### Why is the Universal Gas Constant important?

It's essential for calculating relationships between gas properties like pressure, volume, and temperature.

#### Where is R-specific commonly used?

In engineering and specialized calculations for specific gases.

#### How do you calculate R-specific?

Divide the Universal Gas Constant by the molar mass of the gas.

#### Is R used in both chemistry and physics?

Yes, it's fundamental in both fields for studying gases.

#### What role does R-specific play in engineering?

It's used for precise calculations in gas systems and processes.

#### How does molecular weight affect R-specific?

Higher molecular weight gases have a lower R-specific value.

#### Does R change with different gases?

No, R is a universal constant for all ideal gases.

#### Is the Universal Gas Constant the same in all unit systems?

Its value remains constant, but the units may differ (e.g., J/(mol·K), L·atm/(mol·K)).

#### Can R be used for real gases?

It's primarily for ideal gases, but with corrections, it can be applied to real gases under certain conditions.

#### Are both constants used in thermodynamics?

Yes, both are used, depending on whether the focus is on general or specific gas properties.

#### Why might an engineer choose R-specific over R?

For calculations requiring specific information about a particular gas, R-specific provides more accuracy.