Answer
$3.7\times10^{-4}\,g/L$
Work Step by Step
Recall: Density $d= \frac{PM}{RT}$ where P is the pressure, M is the molar mass, R is the universal gas constant and T the temperature.
Given:
$P= 0.20\,mmHg\times\frac{1 \,atm}{760\, mmHg}$
$=2.6\times10^{-4}\,atm$
$T= (-23+273) K=250 K$,
$R=0.0821\,L\,atm\,mol^{-1}K^{-1}$ and
$M= 29.0\,g/mol$
Substitute: $d=\frac{(2.6\times10^{-4}\,atm)(29.0\,g/mol)}{(0.0821\,L\,atm\,mol^{-1}K^{-1})(250\,K)}$
$=3.7\times10^{-4}\,g/L$
