Answer
$7.5\times 10^4\frac{g}{mol}$
Work Step by Step
As $M=\frac{\pi}{RT}$
We plug in the known values to obtain:
$M=\frac{18.6torr\times \frac{1atm}{760torr}}{0.08206\frac{L.atm}{mol.K}\times 298K}=1.00\times 10^{-3}\frac{mol}{L}$
Now $molar \space mass=\frac{0.15g}{0.0020L\times 1.00\times 10^{-3}\frac{mol}{L}}=7.5\times 10^4\frac{g}{mol}$
