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Abnormal Molar Masses

Molar mass that is either lower or higher than the expected or normal value is called as abnormal molar mass.

Van’t Hoff introduced a factor i, known as the van’t Hoff factor, to account for the extent of dissociation or association. This factor i is defined as:

Ratio of normal molecular mass to the observed molecular mass of the solute.

straight i space equals space fraction numerator Normal space molar space mass over denominator Abnormal space molar space mass end fraction
space equals space fraction numerator Observed space colligative space property over denominator Calculated space colligative space property end fraction

Van’t Hoff factor modifies the equations for colligative properties as follows:

1 right parenthesis thin space fraction numerator straight p subscript 1 superscript straight o minus straight p subscript 1 over denominator straight p subscript 1 superscript straight o end fraction space equals space straight i. straight n subscript 2 over straight n subscript 1
2 right parenthesis space increment straight T subscript straight b space equals space iK subscript straight b straight m
3 right parenthesis thin space increment straight T subscript straight f space equals space iK subscript straight f straight m
4 right parenthesis space straight pi space equals space fraction numerator begin display style straight i space straight n subscript 2 RT end style over denominator straight V end fraction

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