Molality is simply defined as the number of moles of
solute over the mass of the solvent.
In this case the solute is NaCl and the solvent is water,
therefore:
moles solute = 0.630g/ 58.44 g/mol = 0.0108 mol
So the molality is:
m = 0.0108 mol / (0.525 kg)
m = 0.0205 molal
