Answer
129 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= 1 atm, T= 273.15 K, $R=0.0821\,L\,atm\,mol^{-1}K^{-1}$ and
$d=5.75\,g/L$
Rearrange and substitute: $M=\frac{dRT}{P}=\frac{(5.75\,g/L)(0.0821\,L\,atm\,mol^{-1}K^{-1})(273.15\,K)}{1 \,atm}=129\,g/mol$
