Answer
$M=44.0\frac{g}{mol}$
Work Step by Step
We know that
$PV=nRT$
But $n=\frac{m}{M}$ Where $m$ and$M$ are mass in grams and molar mass respectively
$\implies PV=\frac{mRT}{M}$
This can be rearranged as
$M=\frac{mRT}{PV}$...........eq(1)
$T=28+273.15=301.15K$
$P=745mmHg\times\frac{1atm}{760mmHg}=0.98026atm$
We plug in the known values in eq(1)to obtain:
$M=\frac{0.433\times 0.08206\times 301.15}{0.98026\times 248\times 10^{-3}}=44.0\frac{g}{mol}$
