x % w / v solution of urea is isotonic with 4 % w / v solution of a non-volatile solute of molar mass 120…

Two solutions having same osmotic pressure at a given temperature are called isotonic solutions. When such solutions are separated.

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

$\mathrm{\pi }=$Osmotic pressure

C=Concentration in molarity

R=Universal gas constant

T=Temperature

As the solutions are isotonic

$$\begin{gathered} {\mathrm{\pi }}_{\mathrm{urea}}={\mathrm{\pi }}_{\mathrm{X}} \\ {\mathrm{C}}_{\mathrm{urea}}={\mathrm{C}}_{\mathrm{X}} \end{gathered}$$

X % solution of urea means X grams of urea in $100$ mL of the solution.

$4$% non-volatile solute contains 4 grams of solute in $100$mL of the solution

$$\begin{gathered} {\left[\frac{{\mathrm{W}}_{\mathrm{urea}}}{{\mathrm{M}}_{\mathrm{urea}}}\times \frac{1000}{\mathrm{V}}\right]}_{\mathrm{urea}}={\left[\frac{{\displaystyle {\mathrm{W}}_{}}}{{\displaystyle {\mathrm{M}}_{}}}\times \frac{{\displaystyle 1000}}{{\displaystyle \mathrm{V}}}\right]}_{} \\ {\left[\frac{{\displaystyle \mathrm{X}}}{{\displaystyle 60}}\times \frac{{\displaystyle 1000}}{{\displaystyle 100}}\right]}_{\mathrm{urea}}={\left[\frac{{\displaystyle 4}}{{\displaystyle 120}}\times \frac{{\displaystyle 1000}}{{\displaystyle 100}}\right]}_{} \\ \mathrm{X}=2\mathrm{g} \end{gathered}$$

Hence, the value of X is $2\mathrm{g}$