c
Let $C_V$ and $C_P$ be molar specific heats of the ideal gas at constant volume and constant pressure,
respectively, then
${C_P} = M{c_p}\,and\,{C_V} = M{c_v}$
${C_p} - {C_V} = R\,\,\therefore \,\,M{c_p} - M{c_v} = R$
$ \Rightarrow {c_p} - {c_v} = R/M$