Answer
121 g/mol
Work Step by Step
Recall: $M=\frac{dRT}{P}$ where $d$ is the density, $P$ is the pressure, $M$ is the molecular weight, $R$ is the universal gas constant and $T$ the temperature in K.
Given/Known: $P=790.\,torr\times\frac{1\,atm}{760\,torr}$
$=1.039474\,atm$
$R=0.0821\,L\,atm\,mol^{-1}K^{-1}$
$T=(200.+273)K= 473\,K$
$d=\frac{26.8\,g}{8.29\,L}=3.232811\,g/L$
Substitute:
$M=\frac{(3.232811\,g/L)(0.0821\,L\,atm\,mol^{-1}K^{-1})(473\,K)}{1.039474\,atm}$
$=121\,g/mol$
