The heat capacity of 12.0 g of helium is about

Amount of helium, $n=\frac{m}{M}=\frac{12\mathrm{g}}{4\mathrm{g}{\mathrm{mol}}^{-1}}=3\mathrm{mol}$. Heh um 1s monatomic gas. Its $C_{V,\mathrm{m}}=(3/2)R$ Heat capacity, $C_V=n{C}_{V,\mathrm{m}}=\left(3\mathrm{mol}\right)(3/2)R=\left(\frac{9}{2}\mathrm{mol}\right)\left(8.314{\mathrm{JK}}^{-1}{\mathrm{mol}}^{-1}\right)=37.41{\mathrm{JK}}^{-1}$