L10(13.5) - Using Colligative Properties to Find the Solute...

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Using Colligative Properties to Find the Solute Molar Mass The molar mass of the solute can be obtained by measuring one of the colligative properties of the solution, the mass of the solute and the mass of the solvent (or the volume of the solution) P and T b are not very sensitive to changes in the solute concentration (rarely used for molar mass determinations) T f is more sensitive especially when solvents with large k f constants are used Π is most sensitive and can be used for substances with low molar solubility such as large biomolecules Example: The T f of camphor is 179.80 ° C and it’s k f is 39.7 ° C / m . When 200.0 mg of a compound (X) are added to 100.0 g of camphor, it’s freezing point drops to 179.29 ° C . What is the molar mass of X? Calc. the m of X: ( ) kg mol 013 0 C/m 39.7 C 179.29 - 179.80 . k T m m k T f f f f = ° ° = = = Calc. the number of moles of X: mol 0.0013 kg 1000 0 kg mol 0.013 = × = . ) camphor ( kg ) X ( mol m Calc. the molar mass of X: g/mol 160 mol 0.0013 g 0.2000 = = ) X ( mol ) X ( grams MW Example: A 3.0 g
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