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Abnormal molar mass and van't Hoff factor

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Problem

When 20 g of naphthoic acid (C11H8O2) is dissolved in 50 g of benzene, a freezing point depression of 2K is observed.
(Kf=1.72 K kg/mol)
What will be the van ’t Hoff factor (i)?
  • Your answer should be
  • a simplified improper fraction, like 7/4
  • an exact decimal, like 0.75