The ideal gas constant, symbolized as R, is a fundamental constant in the ideal gas law and can be found by understanding the relationship between pressure, volume, temperature, and the number of moles of a gas.
Understanding the Ideal Gas Constant
The ideal gas constant (R) is a proportionality constant that relates the energy scale to temperature and quantity of matter. It appears in the ideal gas law equation:
PV = nRT
Where:
- P = Pressure of the gas
- V = Volume of the gas
- n = Number of moles of the gas
- T = Temperature of the gas (in Kelvin)
- R = Ideal Gas Constant
Common Values of R
The ideal gas constant has different numerical values depending on the units used for pressure, volume, and temperature. The reference provides two common values:
| Value of R | Units |
|---|---|
| 0.0821 | L • atm / mol • K |
| 8.31 | J / mol • K |
- R = 0.0821 L·atm/mol·K: This value is used when pressure is measured in atmospheres (atm), volume in liters (L), and temperature in Kelvin (K).
- R = 8.31 J/mol·K: This value is used when energy is measured in Joules (J), the volume in cubic meters (m3), and the pressure in Pascals (Pa) – however, it is also frequently used when working with energy, regardless of the initial units for P and V.
How to find the Ideal Gas Constant Practically
While the value of R is a constant, it is good to know how it was experimentally determined. Historically, experiments were conducted with gases under near-ideal conditions. Using the known values from these experiments and the ideal gas law, the numerical value for R was derived.
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Experimental Approach: By measuring the pressure (P), volume (V), and temperature (T) of a known number of moles (n) of a gas, one can rearrange the ideal gas law equation to solve for R:
R = PV / nT
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Standard Conditions: A common method involves using the volume of one mole of an ideal gas at Standard Temperature and Pressure (STP).
- STP is defined as 0°C (273.15 K) and 1 atm.
- One mole of an ideal gas at STP occupies a volume of 22.414 L.
- Plugging these values into the equation we get:
- R = (1 atm 22.414 L) / (1 mol 273.15 K) ≈ 0.0821 L·atm/mol·K
Using the Ideal Gas Constant
- The ideal gas constant is a universal constant, meaning it applies to all gases as stated in the reference.
- Its value is crucial for various calculations involving gases.
- Be very careful to match the units of R with the units of pressure, volume, and temperature in your calculations.
