For what processes is Mayer's formula applicable?

I know that


$$C\_p - C\_V = R, \label{eqn:1}\tag{1}$$


but I have also seen


$$C\_p - C\_V = R/M \label{eqn:2}\tag{2}$$


being used in case of ideal gas.
In what cases \eqref{eqn:1} is used and in what cases \eqref{eqn:2} is used?




The former difference is for molar heat capacity in J/K/mol, the latter for specific heat capacity J/K/kg, or J/K/g, depending on M unit.


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