If $M$ is molecular mass of the gas, then from

$M\left(C_p-C_v\right)=R$

$M=\frac{8.31}{210}=0.0392$

If $\mathrm{\rho }$ is density of the gas at NTP, then mass of 1 ${\mathrm{m}}^3$of gas at NTP$=\mathrm{\rho }$ kg

$\therefore$ Mass of 22.4 L $(=22.4\times {10}^{-3}{\mathrm{m}}^3)$ of gas at NTP $=\mathrm{\rho }\times 22.4\times {10}^{-3}\mathrm{ }$kg, which is the molecular mass of the gas

$\therefore \mathrm{ }\mathrm{\rho }\times 22.4\times {10}^{-3}=0.0392$

$\mathrm{\rho }=\frac{0.0392}{22.4\times {10}^{-3}}=1.75\mathrm{ }\mathrm{k}\mathrm{g }{\mathrm{m}}^{-3}$