The molality (in $mol k{g}^{-1}$) of 1 mole of solute in 50g of solvent is 

Given,  Moles of solute (n) = 1 mol
Mass of solvent  $\left(w_A\right)=50g$
Molality (m) = $\frac{n\times 100}{w_A\left(ing\right)}$
$\therefore m=\frac{1000}{50}=20molk{g}^{-1}$