The osmotic pressure of a non-electrolytic solution can be calculated as follows,

$\mathrm{\pi }=\mathrm{CRT}$

$\mathrm{\pi }$= osmotic pressure

$\mathrm{C}$ = molarity$=\frac{\mathrm{Number} \mathrm{of} \mathrm{moles}}{\mathrm{Volume} \mathrm{of} \mathrm{solution} \mathrm{in} \mathrm{L}}$

$\left(1.29\times {10}^{-3}\right)=\frac{0.63}{\left(\mathrm{MW}\right)(0.3)}\times (0.083)\times 300$

$\mathrm{MW}=\frac{0.63\times 0.083\times 300}{(0.3)\times \left(1.29\times {10}^{-3}\right)}$

$= 40534.88$

$\approx 40535$ (Nearest integer)