The specific heat at constant volume for the monatomic argon is 0.075 kcal/kg-K, whereas its gram molecular specific heat is Cv = 2.98 cal/mol/K. The mass of the argon atom is (Avogadro's number = 6.02 x 1023 molecules/mol)

Molar specific heat = molecular weight X gram specific heatCv=M×cv2.98cal/mol−K=M×0.075kcal/kg−K =M×0.075×103103cal/g−K∴  Molecular weight of argon      M=2.980.075=39.7g i.e., mass of 6.023×1023 atom =39.7gTherefore, mass of single atom=39.76.023×1023=6.60×10−23g