Question

Which of the following statements is false?
A.Units of atmospheric pressure and osmotic pressure are the same.
B.In reverse osmosis, solvent molecules move through a semipermeable membrane from a region of lower concentration of solute to a region of higher concentration.
C.The value of the molal depression constant depends on the nature of the solvent.
D.The relative lowering of vapor pressure is a dimensionless quantity.

Answer 1

Hint: According to the Van’t Hoff osmotic pressure is a colligative property. On adding non-volatile solute, the osmotic pressure is directly proportional to the molar concentration of solute. And directly proportional to the absolute temperature of the solution.

Complete step by step answer:
The minimum amount of pressure required to stop the process of osmosis is called osmotic pressure. As osmotic pressure is one type of pressure the unit of this will be as the unit of atmospheric pressure. So, option A is true.
The molal depression constant can be defined as when the molality of the solution is 1 the amount of freezing point depression is equal to the molal depression constant. The value of the molal depression constant is different for different solvents. That is, it depends on the nature of the solvent. So, option B is true.
On adding non-volatile solute, the vapor pressure of the solution is observed to be lower than the vapor pressure of the pure solvent. This causes the solid and the solution to reach equilibrium at lower temperatures and that causes freezing point depression.
The relative lowering of vapor pressure is equal to the mole fraction of solute in the solution. Mole fraction is a dimensionless quantity. So, the relative lowering of vapor pressure is a dimensionless quantity. Therefore, option D is also true.
Now In reverse osmosis, solvent molecules move through a semipermeable membrane from a region of higher concentration of solute to a region of lower concentration. Therefore, the option C is not true.

Therefore, the correct option is C.

Note:When a substance starts to freeze, the molecules slow down due to the decreases in temperature and mobility of ions; and the intermolecular forces start to take over. At the freezing point of a solvent, an equilibrium exists between the liquid state and the solid-state of the solvent. This tells that the vapor pressures of both the liquid and the solid phase are equal. Adding a solute decreases the freezing point depression due to the solute-solvent interactions. It is a colligative property. The amount of change in the freezing point is related to the number of particles of solute in a solution and does not depend on the chemical composition of the solute.