posted by Jen on .
You mix 40.0 g of ethanol (C2H5OH, molecular weight = 46.1 g/mol) and 60.0 g of water (molecular weight = 18.0 g/mol) The resulting solution has a density of 920 g/L.
a)Calculate the molarity of ethanol in this solution?
Answer : 0.06 kg of H2O
This is correct.
b) At what temperature will the solution freeze? Pure water freezes at 0.00 degrees C and its Kf= 1.86 degrees C/m. Hint: ethanol is not an ionizing solute.
My work: delta Tf= i*Kf* Msolute
delta Tf= 26.97
=26.97 degrees C
This is correct.
I need help on part c and d:
c) calculate the molarity of ethanol in this solution
d) What pressure needs to be applied to this solution at T= 0 degrees C to make water come out from it through a water-permeable membrane?
You have 40.0 g ethanol in 40+60 = 100 g soln. You know the density of the solution; volume = mass/density and convert to L. You have moles ethanol from an earlier part. M = moles/L soln.
d. pi = MRT
for part c:
100g* 1L/920 g= .108695 L
0.87 mols of H2O/.108695 L
Is this right?
pi= (8.004M) (0.08206) (-26.97+273)= 161.59 atm
is this right?
c looks ok but I don't understand why you have the line 0.87 mols of H2O/.108695. You have moles ethanol already and you have volume already. You don't need this line nor the calculation.
I don't agree with your answer from a. 0.06 kg is mass of solvent but it isn't molarity (or molality).
Also, I don't agree with b. It can't possibly be -26.+ which makes d wrong before I even look at it. And while I'm at it, please note that I discourage you from using i*Kf*Msolute. M means molarity. m means molality and m is what goes there.
sorry I typed it wrong....my answer for a) is 14.5 M
what did you get for part b then?
oohhh so for part d I mixed up molarity and molality my bad...I get it now! So the answer I got in part a is supposed to be used in part d......Thanks! :)
What did you get for part b if you disagree with -26?
No, the number in a is m and the number to be used in d is M. Your 8.00 is ok there. Its the -26.97 that stopped me. And your answer for part a is 14.5 m; i.e., 14.5 molal.
For b part I have
dT = i*Kf*m
dT = 1*0.186*14.46
dT = 2.6898 which I would round to 2.69 which makes the soln freeze at -2.69 C.
But Kf= 1.86 degrees C/m not 0.186? So shouldn't it be :
dT = 1*1.86*14.46 =26.97
You are right and I am wrong. I don't know how that crept into my mind. I've worked a problem like this a million times. Let me look at the last problem.
Okay thanks sooooo much! I really really really appreciate your help! It's fine everyone makes mistakes! :)
I went back and re-read the problem and it says that you want all of that info AT T= 0 degrees C so I think you want 273 for T and not delta T.
With regard to part a, I think I got pushed off the bus on part a when I saw the 0.06 and that skewed my thinking. Sorry about that.
No problem! Thanks! :)