The value of the gas constant RRR when using atmospheres (atm) for pressure is 0.0821 L·atm/(mol·K). This means that in the ideal gas law PV=nRTPV = nRTPV=nRT, if pressure is in atm, volume in liters, and temperature in Kelvin, you can use R=0.0821R = 0.0821R=0.0821 directly.