An isotonic solution of glucose with urea contains 18\ g in 1\ dm^3...

For isotonic solutions, the molar concentrations of the solutes are equal.

$C_{\text{glucose}} = C_{\text{urea}}$

The concentration of the glucose solution is given by:

$C_{\text{glucose}} = \dfrac{18}{180 \times 1} = 0.1\ \text{mol dm}^{-3}$

Let $W$ be the mass of urea in $500\ \text{mL}$ ($0.5\ \text{dm}^3$) of the solution. The concentration of the urea solution is:

$C_{\text{urea}} = \dfrac{W}{60 \times 0.5}$

Equating the concentrations:

$\dfrac{W}{30} = 0.1$

$W = 3\ \text{g}$

Answer: $3\ \text{g}$.