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Solution — Chemistry STD 12 Science — Question

Bihar BoardEnglish MediumSTD 12 ScienceChemistrySolution5 Marks
Question
Define Osmotic Pressure. Prove that Osmotic pressure is a colligative property.
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Answer

Osmotic Pressure: The excess pressure that must be applied to the solution side to prevent the passage of solvent into it through semi-permeable membrane.
Osmotic Pressure is a Colligative Property: Since, osmotic pressure depends upon the molar concentration of solution or number of moles of solute per litre of solution irrespective of nature of solute, therefore, it is a colligative property.
This result is the same as ideal gas equation. Thus, the solute in the dilute solution behavior like a gas and osmotic pressure of the solution is equal to the pressure which the solute would exert if it were a gas at the same temperature and occupies the same volume as that of the solution. This is called Van't Hoff theory of dilute solutions.
Proof: By combining the above three laws i.e., $ \pi \propto \frac{1}{V} $; $ \pi \propto T $ and $ \pi \propto n $, we get
$ \pi \propto \frac{nT}{V} $ or $ \pi V = nRT $
where R is a constant, called solution constant and its value is found to be same as that of the Gas constant. The above equation can be written as,
$ \pi = \frac{n}{V}RT $ or $ \pi = cRT $
where c is concentration of the solution and is measured in moles per litre.

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