Depression in freezing point is a colligative property.Hence, it depends on the number of solute particles.

${\mathrm{\Delta T}}_{\mathrm{f}}={\mathrm{K}}_{\mathrm{f}}\times$m

m=Molality

${\mathrm{\Delta T}}_{\mathrm{f}}=$Freezing point of solvent$-$Freezing point of solution

${\mathrm{\Delta T}}_{\mathrm{f}}=1.0^{\circ }\mathrm{C}$

$$\begin{gathered} {\mathrm{\Delta T}}_{\mathrm{f}}=1.88\times \mathrm{m} \\ 1.0=1.88\times \frac{\mathrm{x}}{78}\times \frac{1000}{500} \\ \mathrm{x}=20.96 \mathrm{g} \end{gathered}$$

Hence, the weight of non-volatile solid is 20.96grams