a
Molar concentration of urea \(=\,\frac {10}{60}\) per \(dm^3\)

Molar concentration of volatile solute solution \(=\,\frac {5}{M}\) per \(100\,ml\) or ,\(\frac {50}{M}\) per \(dm^3\)

For solution of same concentration or, isotonic solution at same temperature

\(\frac {10}{60}\,=\) \(\frac {50}{M}\) or, \(M=300\,g\,mol^{-1}\)