Given value,
Solution of urea $= 10\%$
Solution of non-volatile solute $'x’ = 6\%$
$\because$ Isotonic solution has the same osmotic pressure and hence the same concentration in the solution.
We know that, osmotic pressure
$(\pi) =CRT$
here, $R$ and $T$ are constant
$\because π_1 = π_2$
$∴ C_1RT_1 = C_2RT_2$
$10\%$ solution of urea $= 6\%$ of solution of $'x’$
$\frac{10}{100\times60}=\frac{6}{100\times x}$ (Molecular weight of urea $=60$)
$x = 6 × 6 = 36\, g \,mol^{−1}$
Hence, the atomic mass of solute $'x'$ is $36 \,gmol^{−1}$.