a
Osmotic pressure \((\pi)\) of isotonic solutions are equal.

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 \(C_{1}=C_{2}\)

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

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