Answer
16.0 g/mol
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= 0.965 atm, T= 445.7 K, $R=0.0821\,L\,atm\,mol^{-1}K^{-1}$ and
$d=\frac{mass}{volume}=\frac{0.423\,g}{1.00\,L}=0.423\,g/L$
Rearrange and substitute: $M=\frac{dRT}{P}=\frac{(0.423\,g/L)(0.0821\,L\,atm\,mol^{-1}K^{-1})(445.7\,K)}{0.965\,atm}$
$=16.0\,g/mol$
