Homework Help: AP CHEM

Posted by Jonathan on Sunday, February 10, 2008 at 5:24pm.

I'm currently working on a lab in which I have to make the solution boil at 245 degrees F. The solution will be a mix of ethylene glycol and I have to caluculate the volumes of water and solute needed to make the solution.

here's my work so far.
X= mL C2H4(OH)2
175mL-X=mL H20

water boils @ 100 degrees C
my bp is 118.33 degrees C

given:
kb= .515 degrees C kg/mol
d of C2H4(OH)2 = 1.1088 g/mL

delta Tb = (kb*g*i)/GMW*kg

18.33 degrees c = (.515)*(1.1088)*(1)/(62.08g/mol)*(175-X(.9970)/1000)
18.33 = .571032X/(62.08)(175-X)(.9970)/1000
18.33 = 571.032 X/(62.08)(174.475-.9970X)
18.33 = 571.032X/ (1031.408-61.89376X)

now i'm stuck. i don't understand how to get x by itself from here. is what i've done so far correct?
thank you.

AP CHEM - DrBob222, Sunday, February 10, 2008 at 9:31pm
I got cross eyed looking at all the numbers. Let me break it down. Also, you don't say how much you want to make. I have assumed 100 g water as the solvent.
Delta T = Kb*m (i is 1).
118.33-100 = 0.515*m
solve for molality.

molality = mols solute/kg solvent = mols solute /0.1
solve for moles solute.

mols solute = grams solute/molar mass solute

If you want to use volume for the ethylene glycol instead of weighing it, then volume = mass desired/density

I hope this helps.

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