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CBSE Class 12-science Answered

<div>sir i want to ask what is vanthoff vector as i am not getting it so please can u help me</div>
Asked by abhishek singh | 26 Apr, 2015, 10:35: AM
Expert Answer
If the molecular mass of a substance determined by studying any of the colligative properties comes out to be different than the theoratically expected value, then the substance is said to show abnormal molecular mass.
The various relations for the colligative properties hold good only for dilute solutions which act as ideal solutions. Thus, even when the solute is non-volatile, non-electrolytic, non-associated and non-dissociated, deviations from the theoratically expected values of colligative properties and molecular masses are observed at higher concentrations. This is because at higher concentrations, the moleculaes are so close to each other that they exert appreciable intermolecular forces of attraction resulting into the deviations from ideal behaviour.
Abnormal molecular masses are observed in any one of the following cases:
(a) When the solution is non-ideal, that is, the solution is not dilute.
(b) When the solute undergoes association in the solution.
(c) When the solute undergoes dissociation in the solution.
 
To calculate the extent of dissociation or association, van't Hoff in 1880 introduced a factor 'i' called van't Hoff factor.
van't Hoff factor is defined as the ratio of the experimental value of the colligative property to the calculated value of the colligative property that is van't Hoff factor,
 
begin mathsize 9px style straight i space equals space fraction numerator Experimental space value space of space the space colligative space property over denominator Calculated space value space of space the space colligative space property space when space the space solution space behaves space ideally end fraction space space space space equals space fraction numerator Observed space value space of space the space colligative space property over denominator Normal space value space of space the space same space property end fraction end style
 
Since the clligative proprty is inversely proportional to the molecular mass of the solute, we can also write
 
begin mathsize 9px style straight i equals fraction numerator Normal space left parenthesis calculated right parenthesis space molecular space mass over denominator Abnormal space left parenthesis Observed right parenthesis space molecular space mass end fraction equals straight M subscript straight c over straight M subscript straight o end style
Answered by Prachi Sawant | 27 Apr, 2015, 08:52: AM
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