Chemical Forums
Chemistry Forums for Students => High School Chemistry Forum => Topic started by: carp14 on April 12, 2009, 02:59:35 PM
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A Solution of 0.64g Epinephrine in 22.79 mL of Carbon Tetrachloride causes an elevation of 0.49 C in the boiling point. What is the molar mass of Epinephrine. What is the molar mass of epinephrine?
I know I can look up the molar mass of epinephrine, but can someone please help me solve this using colligative properties?
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Dear carp14;
First solve the equation for mB:
http://en.wikipedia.org/wiki/Boiling-point_elevation#Calculations
Then you can calculate the molar mass from m and mB.
Additionally you can search the Forums for a lot similar questions.
Good Luck!
ARGOS++
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Those instructions are nowhere near specific enough, I'm taking a highschool chemistry class. If you could be more specific, it would be greatly appreciated.
Thnx
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Dear carp14;
So do it stepwise:
1.) Can you re-arrange/solve the formula from the link given for mB?:
∆Tb = Kb * mB ====> mB = . .
2.) Can you identify what you all know to satisfy this formula, and what is missing?
Good Luck!
ARGOS++
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I'm not as lost as I my seem, yes I can rearrange that formula.
∆Tb = Kb * mB can be rearranged to mB = ∆Tb / Kb
I know that molality is moles of solute divided by kilograms of solution.
I know that carbon tetrachloride is covalnt, so it does not dissociate.
Then I started to try and subsitute numbers which is where I got lost. I made ∆Tb 0.49 because it is the change in elevation. Kb, the boiling point elevation constant for carbon tetrachloride I made 4.95. Therefore my molality was 0.09898. Is this correct? Where do I go from here?
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Dear carp14;
Well done!
Now you know that 0.64g in 22.79 ml is equivalent to 0.09898 molal!
But your definition of molality is wrong!
Please use the correct definition and solve the formula from the definition for the molar mass.
("Ask" Wiki for the density of Carbon Tetrachloride.)
Then you will have finished.
Good Luck!
ARGOS++
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Alas, I don't think I quite have it.
I found the desnity of carbon tetrachloride to be 1.5842 g/cm3
I know I need to multiply this by the amount of carbon tetrachloride that I have which is 22.79 ml. Yet I do not know if I conver the ml to L.
Finally, I could not find a problem with my definition of molality. My notes define it as moles of solute divided by liters of solution. Any hints would be appreciated.
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ABOVE is incorrect. I meant KILOGRAMS of solution.
Source: http://wiki.answers.com/Q/How_do_you_calculate_the_molality_of_a_solution
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I apologized for posting so many times, but this is how my train of thought is going.
I researched the definition of molality.
Peraphs it is moles of solute divdided by ONE kilogram of solution
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Dear carp14;
Sorry!, - You did the same mistake again!
It is moles per kg SOLVENT (not Solution!!).
(You may re-read it once again.)
Good Luck!
ARGOS++
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Well, I'm starting to try
Now my equation is.
0.49= 4.95 *m
m= Molality
m= (moles of solvent)/ (1 kg solvent)
0.64 g Epinephrine * (1 mole / 183.294g) = 0.00349 moles
m = (0.00349 moles)/ 1 kg solvent
But I know molality is 0.9898
0.9898 = (0.00349 moles)/ 1 kg solvent
1 kg solvent = 0.00352596484 UNIT
Alas I also have not worked the density of carbon tetrachloride into here.
Where do I go form here?
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Dear carp14;
You are running into the wrong direction!
Think about that you will find a different molecular weight for Epinephrine from Wiki !!!!
MolaLity as formula: molality = mSolute / MWSolute / mSolvent.
Can you now solve the formula for the MWSolute?
Do you see now that the density is required?
Good Luck!
ARGOS++
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Do the slash marks mean divided by?
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Do the M's mean moles?
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Dear carp14;
Of course!, - slashes stands for division and mx is the "sign" for masses.
Good Luck!
ARGOS++
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molality = mSolute / MWSolute / mSolvent.
That formula is confusing to me. What is the difference between molecular weight and mass? I can not find the mass of epinephrine, unless it is the molar mass, which is the same as molecular weight.
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Dear carp14;
Think about that your mass of Epinephrine is = 0.64 grams, and your mass of Carbon Tetrachloride is = 22.79 ml times the density.
And: mSolute / MWSolute is nothing else than the number of moles!
I think that this will help you.
Good Luck!
ARGOS++
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molality = mSolute / MWSolute / mSolvent.
I used some dimensional analyis.
0.64g Epinephrine * (1 mole/ MW Epinephrine) = 64/x
let x = MW Epinephrine
0.9898= (64/x)/ (22.7*1.5842)
35.5945x=64
x=1.79803059
Where did I go wrong?
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Dear carp14;
You are very close!, - you did only a miner mistake:
You forgot only that you must insert the mass of Solvent in kg (from the molality definition!):
22.79 ml * 1.5842 g/ ml = 36.10g = 0.03610kg
If you use it this way you will get the result if you use 0.64g and not 64!!
And please don't round intermediate results!
Good Luck!
ARGOS++
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I replaced all the numbers and still did not find the correct molecular weight,
My equation now looks like this.
0.9898= (0.64/x)/0.03610
0.0357= (0.64/x)
0.0357x= 0.64
x= (.64/0.0357)
x=17.9271709
Thats not rite!
Where did I Go wrog now?
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Dear carp14;
Please 0.09898 mole/kg and not 0.9898 mole/kg!!
Then it will come fine!
Good Luck!
ARGOS++
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At this point, I need to thank you as you have been more than extremely helpful!
Yet it is bugging me, I solve the equation to 179.11, without any rounding.
The molar mass fo epinephrine is 183.204, why is this?
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Don't even answer that.
My boiling point elevation is wrong.
I appreciate all your *delete me*
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Correct Boiling point elevation is 5.03
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Dear carp14;
I never believe that!, - Maybe 0.503°K could be possible!
Good Luck!
ARGOS++
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I have no idea what you just said. :)
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I meant the molal boiling point elevatio constant was wrong, and should be 5.03.
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Dear carp14;
That sounds different!, - But Wikipedia listed 4.95 °K / molal.
Good Luck!
ARGOS++