Molality ($m$) is defined as the number of moles of solute dissolved per kilogram (1000 g) of the solvent . $\text{Molality (m)} = \frac{\text{Moles of solute}}{\text{Mass of solvent in kg}}$ Given: Molality ($m$) = $1 \text{ m} = 1 \text{ mol kg}^{-1}$ Moles of solute ($n$) = $0.5 \text{ mol}$ Substituting the values: $1 = \frac{0.5}{\text{Mass of solvent in kg}}$ $\text{Mass of solvent in kg} = 0.5 \text{ kg}$ Since $1 \text{ kg} = 1000 \text{ g}$, the mass of the solvent is $0.5 \times 1000 \text{ g} = 500 \text{ g}$.