In one molal solution that contains $0.5\text{mole}$ of a solute, there is:

Molality ($m$) is defined as the number of moles of solute per kilogram of solvent.
$$m=\frac{\text{moles of solute}}{\text{mass of solvent (kg)}}$$
Given $m=1\text{mol/kg}$ and moles $=0.5\text{mol}$:
$$1=\frac{0.5}{x}⟹x=0.5\text{kg}$$
$0.5\text{kg}$ of solvent is equal to $500\text{g}$ of solvent.