I’m confused using the gas constant R. When finding Kp, R is 0.0821 but in few other equations, R is 8.314 . How do I know when to use which value for R
The Correct Answer and Explanation is:
The gas constant, R, is used in various thermodynamic equations, and its value depends on the units involved in the specific equation. Understanding when to use R = 0.0821 L·atm/mol·K versus R = 8.314 J/mol·K hinges on the units of pressure, volume, and temperature.
- R = 0.0821 L·atm/mol·K is typically used when dealing with equations involving pressure in atmospheres (atm) and volume in liters (L). This value of R is commonly applied in the ideal gas law: PV=nRTPV = nRTPV=nRT where:
- P = pressure in atmospheres (atm)
- V = volume in liters (L)
- n = number of moles of gas
- R = 0.0821 L·atm/mol·K
- T = temperature in Kelvin (K)
- R = 8.314 J/mol·K is used when the units of pressure are in pascals (Pa) and the volume is in cubic meters (m³). This is a more universal form of the gas constant and is often used in thermodynamic equations, especially those related to energy and work in the context of gases. For example: PV=nRTPV = nRTPV=nRT where:
- P = pressure in pascals (Pa)
- V = volume in cubic meters (m³)
- n = number of moles
- R = 8.314 J/mol·K
- T = temperature in Kelvin (K)
- When finding Kp (equilibrium constant in terms of partial pressures), you use R = 0.0821 L·atm/mol·K when the pressures are given in atmospheres. This is because the equilibrium constant Kp relates the pressures of gases in the reaction to each other and is typically expressed in atmospheres (atm).
In summary, the value of R you use depends on the units of pressure and volume in your equation. If working with atmospheres and liters, use 0.0821. If using pascals and cubic meters, use 8.314. The unit consistency ensures correct calculations in thermodynamics.
