The freezing point of an aqueous solution of glucose was measured to be −1.52°C. What is the osmotic pressure, in atm, of 100 mL of this solution at 25.0°C? (Kf = 1.86°C/m; dH20 = 1.00 g/mL) (Ans. 20.0 atm)

When I go from molality to molarity how come I don't figure out the total volume of the solution and divide it by the number of moles from molality? I don't get how he just went from moles/ kg of sold to M. The only way to do that is by saying the the glucose doesn't contribute to the volume of the solution but I guess thats what the density is saying.

Question

The freezing point of an aqueous solution of glucose was measured to be −1.52°C. What is the osmotic pressure, in atm, of 100 mL of this solution at 25.0°C? (Kf = 1.86°C/m; dH20 = 1.00 g/mL) (Ans. 20.0 atm)

When I go from molality to molarity how come I don't  figure out the total volume of the solution and divide it by the number of moles from molality? I don't get how he just went from moles/ kg of sold to M. The only way to do that is by saying the the glucose doesn't contribute to the volume of the solution but I guess thats what the density is saying.

anonymous · Answer

It's a pretty reasonable assumption, if you need to make it.  Glucose will form a pretty ideal solution with water, which means the volume of the solution will be close to just the volume of water used to make it.