Lake Vida is an enormous hypersaline lake that lies under 19 m of ice in the Dry Valleys region of Antartica. The freezing point of the water is approximately -17 C owing to the fact that it is many times as salty as typical saltwater (assume all of the salt is NaCl). Calculate the osmotic pressure of this solution just at the freezing point. (the density of this hypersaline brine is 1.11g/mL)
I would try something like this.
delta T = i*Kf*molality
i = 2 for NaCl.
Substitute and solve for molality.
You have the density, convert molality to molarity, then p = MRT