Posted by determining molar mass on Monday, July 11, 2011 at 10:39pm.
Okay so I know how I would normally determine molar mass of a compound *get out periodic table, sum up elements present, eg. H20 = [2(1.008)+16.00]g/mol*
But this is for a lab class and they want us to derive it.
Info found: Ethylene glycol (C2H6O2 according to book) dissolved in water:
mass water: 150 g
mass ethylene glycol: 50 g
found boiling point: 103 C
MW ethylene glycol: ?
We've been using the boiling point elevation equation a lot
I suppose the molality would be:
1.) 50g solute*(1 mole solute/62.068g solute)=0.81 mol solute
2.) 150g solvent*(1 kg solvent/1000 g solvent)=0.150kg solvent
3.) m = 0.81 mol solute/0.150kg solvent
m = 5.4
van't Hoff factor of ethylene glycol is (I believe) 1 (organic compound)
this would make k(boil)=^T(boil)/m or more specifically ~0.74 (technically 20/27 but that would violate sig. figs)
but I'm not sure what any of this has to do w/ calculating the molecular weight.
Thank you in advance.
chemistry - DrBob222, Monday, July 11, 2011 at 11:28pm
delta T = Kb*m
Solve for m
molality = moles/kg solvent
molality from above x 0.150 = moles ethylene glycol. Solve for moles.
moles = grams/molar mass
You have moles and grams, solve for molar mass. Assuming 103 is your best T measurment, I come up with about 57. The molar mass from the periodic table is about 62. Error about 8%.
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