Question

The spontaneous flow of the solvent through a semipermeable membrane from a pure solvent to а solution or from a dilute solution to a concentrated solution is called osmosis. The phenomenon of osmosis can be demonstrated by taking two eggs of the same size. In an egg, the membrane below the shell and around the egg material is semipermeable. The outer hard shell can be removed by putting the egg in dilute hydrochloric acid. After removing the hard shell, one egg is placed in distilled water and the other in a saturated salt solution. After some time, the egg placed in distilled water swells-up while the egg placed in salt solution shrinks. The external pressure applied to stop the osmosis is termed as osmotic pressure (a colligative property). Reverse osmosis takes place when the applied external pressure becomes larger than the osmotic pressure.

1. Define reverse osmosis. Name one SPM which can be used in the process of reverse osmosis.  [2]
2. 1. What do you expect to happen when red blood corpuscles (RBC’s) are placed in 0.5% NaCl solution?  [1]
                                                      OR
   2. Which one of the following will have a higher osmotic pressure in 1 M KCl or 1 M urea solution? Justify your answer.  [1]
3. Why is osmotic pressure a colligative property?  [1]

Answer 1

(a) Reverse osmosis is a water purification process in which pressure is applied to force water through a semi-permeable membrane, allowing water molecules to pass while removing dissolved salts, impurities, and contaminants.

Thin Film Composite (TFC) membranes are among the most widely used semi-permeable materials in reverse osmosis systems.

(b) (i) When red blood cells (RBCs) are placed in a 0.5% NaCl solution, they absorb water by osmosis, causing them to swell. If excessive water enters the cells, the cell membrane can rupture, resulting in haemolysis (bursting of the cells).
                                                       OR

(ii) The osmotic pressure of a solution is determined by the number of solute particles present, as explained by Raoult’s law and the van’t Hoff equation:

π = iCRT

For KCl, i =2

For urea, i = 1

Hence, the osmotic pressure of KCl will be high.

(c) Osmotic pressure is regarded as a colligative property because it is determined only by the number of solute particles present in a solution and does not depend on the nature or identity of those solute particles.