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Explain abnormal molecular masses. |
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Answer» Solution :When the observed molecular masses obtained from their colligative properties of the SUBSTANCES are different (higher or lower) than the theoretical or normal values calculated from their molecular formulae, then they are called abnormal molecular masses. Explanation : The colligative property depends on the number ofsolute particles in the solution but it is independent of their NATURE. Abnormal values of them arise when the dissolved solute UNDERGOES a molecular change like dissociation or association in the solution. The observed colligative property (or abnormal colligative property) may be higher or lower than the theoretical value. (i) Dissociation of thesolutemolecules : Whena solute likeans electrolyte isdissolvedin a polarsolventlikewater,it undergoesdissociation, whenresult in theincreaseinthenumberof particles inthe solution. Hence, the observed value of the colligative property BECOMES higher than the theoretical value, e.g., when one mole of KCl is dissolved in the solution then due to dissociation, `KCl toK^(+) +Cl-`, the number of particles increases, hence, the colligative properties like osmotic pressure, elevation in the boiling point, ETC. increase. (ii) Association of the solute molecules , When a solute like a nonelectrolyte is dissolved in a nonpolar solvent like benzene, it undergoes association forming molecules of higher molecular mass. Hence, the number of the particles in the solution decreases. Therefore the colligative properties like osmotic pressure, elevation in the boiling point, etc. are lower than the theoretical value, e.g.,`nA to An`. `2CH_(3)COOH to (CH_(3)COOH)_(2) 2C_(6)H_(5)COOH to (C_(6)H_(5)COOH_(2))` |
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