Calculate the molar mass of an unknown solid (assume a non-electrolyte) if 1.35

Question

Calculate the molar mass of an unknown solid (assume a non-electrolyte) if 1.35 grams of the solid was dissolved in 15.0 grams of dodecanol and caused a freezing point depression of 2.34 Celsius. Calculate the molar mass of this compound if it were actually an ionic compound that dissociates into two parts.

Explanation / Answer

1) Calculate the molar mass of an unknown solid (assume a non-electrolyte) if, 1.35 grams of the solid was dissolved in 15.0 grams of dodecanol and caused a freezing point depression of 2.34 degrees C. when this question was asked in 2009 @http://answers.yahoo.com/question/index?… they supplied the freezing point depression constant for dodecanol as 2.9C/molal dT = Kf (molality) 2.34 C = (2.9C/molal) (molality) molality = 2.34 / 2.9 = 0.8069 molal using that molality, find the moles within the 15.0 g of solvent 15.0 g solvent @ 0.8069 moles unknown / 1000 g soalvent = 0.0121 moles of unknown now find the molar mass of the unknown 1.35 grams / 0.0121 moles = 111.54 grams /mole your answer rounded to 3 sig figs is 112 g/mol ======================================… and then 2) Calculate the molar mass of this compound if it were actually an ionic compound that dissociates into 2 parts. (i.e. BX --> (B+) + (X-) by the equation BX --> (B+) + (X-) 0.0121 moles was released from 1/2 as many moles of molecules = 0.00605 moles now find the molar mass of the unknown 1.35 grams / 0.00605 moles = 223.14 grams /mole your answer rounded to 3 sig figs is 223 g/mol

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