The Universal Gas Constant & its Use
The Universal Gas Constant, R, is a constant used to bring two sides of a mathematical expression into equality. I often refer to the universal gas constant as a fudge factor.
For experiments (and problems) using pressures, the value of R is 0.0821 and the units are L atm/mol K
R= 0.0821 L atm/mol K
For experiments (and problems) using concentrations, the value of R is 8.3145 and the units are J/mol K. More properly the units should be kJ/mol K and the value of R would then be 0.00831.
R=8.3145 J/mol K
R=0.00831 kJ/mol K
The difference in values is caused by the change in units.
When you are beginning a problem which might involve an R value, look at the units of the other parts of the question to see which R value to use. I have given a reasonable rule of thumb above for pressures and for concentrations. If you have a mixed calculation where both pressures and concentrations will be used, the pressure values are generally converted to concentrations using PV=nRT.