For solution of unknown substance $(\pi=C R T)$

$C_{1}=\frac{5.25 / M}{V}$

For solution of urea,

$C_{2}$ (concentration) $=\frac{1.5 / 60}{V}$

Given, $\pi_{1}=\pi_{2}$

as $\quad \pi=C R T$

$\therefore \quad C_{1} R T=C_{2} R T$ or $C_{1}=C_{2}$

$\quad \frac{5.25 / M}{V}=\frac{1.8 / 60}{V}$

$\therefore \quad M=210 \;g / m o l$