CBSE Class 12 · 2008 · All India · SolutionsShort Answer

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Define the term osmotic pressure. Describe how the molecular mass of a substance can be determined on the basis of osmotic pressure measurement.

Answer: π = excess pressure that just stops osmosis. Measure π of a solution of known mass concentration; then M = wRT/(πV).

Solution

Osmotic pressure (

π

) is the minimum excess pressure that must be applied on a solution to just stop the entry of solvent into it through a semipermeable membrane.
For a dilute solution,

π = C R T = \frac{n}{V} R T = \frac{w}{M V} R T

Dissolve a known mass

w

of the substance to make volume

V

of solution, measure

π

at temperature

T

, and obtain the molar mass from

M = \frac{w R T}{π V}

CBSE Class 12 Board · 2008 · All India · Subjective

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