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ta123 Group Title

The normal boiling point of pure benzene is found to be 80.10 degree Celsius. What is the approximate molecular weight of a non-ionizing substance if a solution of 3.55 g of the substance dissolved in 100.0 g of benzene has a boiling point of 80.19 degree Celsius? Kb for benzene (C6H6)= 5.12 degree Celsius/m

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  1. ta123 Group Title
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    need help

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  2. Carl_Pham Group Title
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    You are being asked to work backward from boiling point elevation (one of the colligative properties) to the molar mass of the solute. First, you'll need to use the rule for boiling point elevation, which is that dT_b = K_b m, where dT_b is the boiling point elevation, m is the molality of the solute (moles solute per kg of solvent), and K_b is a constant specific to each substance. (Fortunately you're given K_b for benzene here.) Now you know the molality of the solute. From the mass of solvent, and the definition of molality, you can calculate the moles of solute that were dissolved in the benzene. Finally, since you're told the mass of the solute, and now you know how many moles that was, you can calculate the molar mass (grams per mole) of the solute.

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