JEE Main & Advanced

Molecular masses can be calculated by measuring any of the colligative properties. The relation between colligative properties and molecular mass of the solute is based on following assumptions. (1) The solution is dilute, so that Raoult’s law is obeyed. (2) The solute neither undergoes dissociation nor association in solution. In case of solutions where above assumptions are not valid we find discrepencies between observed and calculated values of colligative properties. These anomalies are primarily due to (i) Association of solute molecules. (ii) Dissociation of solute molecules. (i) Association of solute molecules : Certain solutes in solution are found to associate. This eventually leads to a decrease in the number of molecular particles in the solutions. Thus, it results in a decrease in the values of colligative properties. Colligative property\[\propto \frac{1}{\text{molecular mass of solute}}\]  therefore, higher values are obtained for molecular masses than normal values for unassociated molecules. (ii) Dissociation of more...

The colligative properties of solutions, viz. lowering of vapour pressure, osmotic pressure, elevation in b.p. and depression in freezing point, depend on the total number of solute particles present in solution. Since the electrolytes ionise and give more than one particle per formula unit in solution, the colligative effect of an electrolyte solution is always greater than that of a non-electrolyte of the same molar concentration. All colligative properties are used for calculating molecular masses of non-volatile solutes. However osmotic pressure is the best colligative property for determining molecular mass of a non-volatile substance. Colligative properties are depending on following factory (1) Colligative properties \[\propto \] Number of particles                                       \[\propto \] Number of molecules (in case of non-electrolytes)                                       \[\propto \] Number of ions (In case of electrolytes)                                      \[\propto \] Number of moles of solute                                      \[\propto \] Mole fraction of solute (2) For different solutes of same more...

Freezing point is the temperature at which the liquid and the solid states of a substance are in equilibrium with each other or it may be defined as the temperature at which the liquid and the solid states of a substance have the same vapour pressure. It is observed that the freezing point of a solution is always less than the freezing point of the pure solvent. Thus the freezing point of sea water is low than that of pure water. The depression in freezing point  \[(\Delta T\] or \[\Delta {{T}_{f}})\] of a solvent is the difference in the freezing point of the pure solvent \[({{T}_{s}})\] and the solution \[({{T}_{sol.}})\].                      \[{{T}_{s}}-{{T}_{sol}}=\Delta {{T}_{f}}\] or \[\Delta T\] \[NaCl\] or \[CaC{{l}_{2}}\] (anhydrous) are used to clear snow on roads. They depress the freezing point of water and thus reduce the temperature of the formation of ice. Depression in freezing point is determined more...

Boiling point of a liquid may be defined as the temperature at which its vapour pressure becomes equal to atmospheric pressure, i.e., 760 mm. Since the addition of a non-volatile solute lowers the vapour pressure of the solvent, solution always has lower vapour pressure than the solvent and hence it must be heated to a higher temperature to make its vapour pressure equal to atmospheric pressure with the result the solution boils at a higher temperature than the pure solvent. Thus sea water boils at a higher temperature than distilled water. If Tb is the boiling point of the solvent and T is the boiling point of the solution, the difference in the boiling point (DT or D Tb) is called the elevation of boiling point.                      \[T-{{T}_{b}}=\Delta {{T}_{b}}\] or \[\Delta T\] Elevation in boiling point is determined by Landsberger’s method and Cottrell’s method. Study of elevation in boiling point more...

(1) Osmosis : The flow of solvent from pure solvent or from solution of lower concentration into solution of higher concentration through a semi-permeable membrane is called Osmosis. Osmosis may be divided in following types, (i) Exo-Osmosis : The outward osmotic flow of water from a cell containing an aqueous solution through a semi-permeable membrane is called as Exo-osmosis. For example, egg (after removing hard shell) placed in conc. NaCl solutions, will shrink due to exo-osmosis. (ii) Endo-osmosis : The inward flow of water into the cell containing an aqueous solution through a semi-permeable membrane is called as endo-osmosis. e.g., an egg placed in water swells up due to endo-osmosis. (iii) Reverse osmosis : If a pressure higher than osmotic pressure is applied on the solution, the solvent will flow from the solution into the pure solvent through the semi-permeable membrane. Since here the flow of solvent is in the more...