Question

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
^T(boil)=i*k(boil)*molality(m)

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.

Answer 1

delta T = Kb*m
3= 0.512*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%.