The specific heat at constant volume for the monatomic argon is 0.075 kcal/kg-K, whereas its gram molecular specific heat is C_v = 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 heat

$\begin{array}{l}{C}_v=M\times c_v\\ \begin{array}{rl}2.98\mathrm{cal}/\mathrm{mol}-\mathrm{K}& =M\times 0.075\mathrm{kcal}/\mathrm{kg}-\mathrm{K}\\ & =M\times \frac{0.075\times {10}^3}{{10}^3}\mathrm{cal}/\mathrm{g}-\mathrm{K}\end{array}\end{array}$

$\therefore \text{ Molecular weight of argon }$

     $\mathrm{M}=\frac{2.98}{0.075}=39.7\mathrm{g}$

$\text{ i.e., mass of }6.023\times {10}^{23}\text{ atom }=39.7\mathrm{g}$

Therefore, mass of single atom

$=\frac{39.7}{6.023\times {10}^{23}}=6.60\times {10}^{-23}\mathrm{g}$