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I found a question in a biochemistry book.

How can osmosis be used to measure molecular weight of macromolecules? Is it applied practically?

A general answer without the usage of many technical terms would be most suitable for me.

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    $\begingroup$ Maybe this helps: brooklyn.cuny.edu/bc/ahp/ChemInvest/CI.MMOsmosis.html $\endgroup$ – Philipp Sep 10 '13 at 16:01
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    $\begingroup$ Out of curiosity... are you expecting a General Chemistry level answer or a Physical Chemistry level answer? $\endgroup$ – Ben Norris Sep 10 '13 at 16:53
  • $\begingroup$ I am a "lay man" in terms of chemistry. I found the question in a biochemistry book. I think a general chemistry answer using not many technical terms would be best :) $\endgroup$ – biogirl Sep 10 '13 at 17:42
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It has to do with the fact, that osmotic pressure of any solution is dependent only on the relative number of particles in the solution (in case of dilute solutions), irrespective of their nature. Hence osmotic pressure is called a colligative property.

Therefore, you take a certain known weight of the substance you want to calculate the molecular mass of, prepare a solution and measure the osmotic pressure. Then you compare this to the known osmotic pressure of standard solution which tells you the relative number of particles required to create such a pressure. From this and the knowledge of the weight you used to prepare the solution, you can get the required molecular weight (weight of Avogadro's number of particles). In practice, certain complications, like dissociation and association might complicate the matter a bit.

This method is widely used to calculate the molecular weight (especially) of polymers. See Berkeley and Hartley method.

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The relationship of osmotic pressure and molecular weight is given by the equation

$$\dfrac{\pi}{C}=\dfrac{\mathrm{R}T}{M_r}$$

In a graph of $\frac{\pi}{C}$ against $C$ the intercept is

$$\mathrm{intercept}=\dfrac{\mathrm{R}T}{M_r}$$

where

  • $C=$ concentration
  • $\mathrm{R}=$ gas constant
  • $T=$ absolute temperature
  • $M_r=$ molecular weight
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  1. Osmotic pressure is measured at room temperature while other colligative properties are measured at high temperatures.

  2. OP uses molarity while other CP uses molality.

  3. OP is used at standard pressure i.e. 1atm.

  4. it can also measure molecular mass for a diluted solution.

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