Solution.

Ocean water was described as containing approximately one NaCl per 100 water molecules, or one mole of NaCl per 100 moles of H₂O. Since the molecular weight of water is 18.0, this is equivalent to one mole of NaCl for every 1800 g of water, or 1.0/1.8 = 0.556 mole of NaCl per kilogram of water. Because each mole of NaCl yields two moles of ions, the effective molality of all ions is twice this, or 1.11 molal, and the freezing point depression is

D T_f=-1.86(1.11) =-2.06° C

Sea water freezes at two degrees below zero.