The properties of solutions which depend on the number of solute particles and are independent of their chemical identity are called colligative properties. These are lowering of vapour pressure, elevation of boiling point, depression of freezing point and osmotic pressure. The process of osmosis can be reversed if a pressure higher than the osmotic pressure is applied to the solution. Colligative properties have been used to determine the molar mass of solutes. Solutes which dissociate in solution exhibit molar mass lower than the actual molar mass and those which associate show higher molar mass than their actual values. Quantitatively, the extent to which a solute is dissociated or associated can be expressed by van’t Hoff factor i. This factor has been defined as ratio of normal molar mass to experimentally determined molar mass or as the ratio of observed colligative property to the calculated colligative property.
Consider the following statements about the osmotic pressure method and about reverse osmosis.
(A) The osmotic pressure method is widely used to determine the molar masses of proteins, polymers and other macromolecules.
(B) In the osmotic pressure method, the pressure is measured around the boiling point of the solvent.
(C) As compared to other colligative properties, the magnitude of osmotic pressure is large even for very dilute solutions.
(D) Cellulose acetate, used as the membrane in reverse osmosis, is permeable to the ions present in sea water but impermeable to water.
Which of the statements given above are correct? Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → (A) and (C) only
Osmotic pressure is measured on a solution kept at about room temperature, and even a very dilute solution gives a measurable value. These features make it the preferred method for large molecules.
(A) The osmotic pressure method is widely used to determine the molar masses of proteins, polymers and other macromolecules. — True. Such substances have large molar masses, so their solutions are very dilute in moles, and osmotic pressure still gives a usable reading.
(B) In the osmotic pressure method, the pressure is measured around the boiling point of the solvent. — False. The measurement is made around room temperature. This suits biomolecules, which are generally not stable at higher temperatures.
(C) As compared to other colligative properties, the magnitude of osmotic pressure is large even for very dilute solutions. — True. This is one of the advantages of the method.
(D) Cellulose acetate, used as the membrane in reverse osmosis, is permeable to the ions present in sea water but impermeable to water. — False. It is the other way round: cellulose acetate is permeable to water but impermeable to the impurities and ions of sea water, so only pure water comes through.
NCERT (2026-27 edition):
1.6.4 Osmosis and Osmotic Pressure – “Measurement of osmotic pressure provides another method of determining molar masses of solutes. This method is widely used to determine molar masses of proteins, polymers and other macromolecules. The osmotic pressure method has the advantage over other methods as pressure measurement is around the room temperature and the molarity of the solution is used instead of molality. As compared to other colligative properties, its magnitude is large even for very dilute solutions.”
1.6.5 Reverse Osmosis and Water Purification – “…A workable porous membrane is a film of cellulose acetate placed over a suitable support. Cellulose acetate is permeable to water but impermeable to impurities and ions present in sea water.”