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posted by  pilot boy on 4/29/2008 8:42:47 PM  |  status: Closed  

Calculating molar mass from molality

Course Textbook Chapter Problem
General Chemistry Chemistry (9th) by Chang 12 N/A
Question Details:
A solution made from 6.50g of an unknown compound in 27.8g of diphenyl has a molality of 0.195 m.  Calculate the molar mass of the unknown in g/mol.
 
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posted by HM on 4/29/2008 9:36:00 PM  |  status: Live
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Well molality is moles of solute/kg of solvent. You know the kg of solvent since 1000 g is a kg and you have 27.8 g you have .0278 kg. Molality is moles of solute/kg of solvent so moles/.0278=.195 so the moles of solute is .005421. Since you have 6.50 g of the compound and molar mass is g/mole just divide g by the moles so 6.5/.005421 which equals 1199.04 g/mole
 
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posted by Jbroncos06balla on 4/29/2008 10:03:17 PM  |  status: Live
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Multiplying the molality by the mass of solvent (in kg) gives moles of unknown solute.  Then, by dividing the mass of solute (in g) by the moles of solute, gives the molar mass of the unknown solute.
 
   ? mol of unknown solute  =  (0.195 mol solute / 1 kg diphenyl) x (0.0278 kg diphenyl)
                                          =  0.00542 mol solute
 
   molar mass of unknown = 6.50 g / 0.00542 mol  =  1.20 x 103 g/mol
 
 
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