Answer
61.7 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:
$P= 715\,mmHg\times\frac{1 \,atm}{760\, mmHg}$
$=0.941\,atm$
$T= (23.0+273) K=296 K$,
$R=0.0821\,L\,atm\,mol^{-1}K^{-1}$ and
$d= 2.39\,g/L$
Rearrange and substitute: $M=\frac{dRT}{P}=\frac{(2.39\,g/L)(0.0821\,L\,atm\,mol^{-1}K^{-1})(296\,K)}{0.941\,atm}$
$=61.7\,g/mol$
