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Topic: Colligative Properties Carbon Tetrachloride Problem  (Read 13278 times)

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Offline carp14

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Colligative Properties Carbon Tetrachloride Problem
« on: April 12, 2009, 02:59:35 PM »
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?

Offline ARGOS++

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Re: Colligative Properties Carbon Tetrachloride Problem
« Reply #1 on: April 12, 2009, 03:24:25 PM »

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++

Offline carp14

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Re: Colligative Properties Carbon Tetrachloride Problem
« Reply #2 on: April 12, 2009, 03:42:28 PM »
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

Offline ARGOS++

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Re: Colligative Properties Carbon Tetrachloride Problem
« Reply #3 on: April 12, 2009, 04:07:19 PM »

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!
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Offline carp14

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Re: Colligative Properties Carbon Tetrachloride Problem
« Reply #4 on: April 12, 2009, 05:58:50 PM »
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?

Offline ARGOS++

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Re: Colligative Properties Carbon Tetrachloride Problem
« Reply #5 on: April 12, 2009, 06:12:23 PM »

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++

Offline carp14

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Re: Colligative Properties Carbon Tetrachloride Problem
« Reply #6 on: April 12, 2009, 06:26:35 PM »
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.

Offline carp14

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Re: Colligative Properties Carbon Tetrachloride Problem
« Reply #7 on: April 12, 2009, 06:27:55 PM »
ABOVE is incorrect. I meant KILOGRAMS of solution.
Source: http://wiki.answers.com/Q/How_do_you_calculate_the_molality_of_a_solution

Offline carp14

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Re: Colligative Properties Carbon Tetrachloride Problem
« Reply #8 on: April 12, 2009, 06:34:26 PM »
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

Offline ARGOS++

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Re: Colligative Properties Carbon Tetrachloride Problem
« Reply #9 on: April 12, 2009, 06:36:32 PM »

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++

Offline carp14

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Re: Colligative Properties Carbon Tetrachloride Problem
« Reply #10 on: April 12, 2009, 06:45:54 PM »
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?

Offline ARGOS++

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Re: Colligative Properties Carbon Tetrachloride Problem
« Reply #11 on: April 12, 2009, 06:54:33 PM »

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++

Offline carp14

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Re: Colligative Properties Carbon Tetrachloride Problem
« Reply #12 on: April 12, 2009, 06:58:22 PM »
Do the slash marks mean divided by?

Offline carp14

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Re: Colligative Properties Carbon Tetrachloride Problem
« Reply #13 on: April 12, 2009, 07:00:22 PM »
Do the M's mean moles?

Offline ARGOS++

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Re: Colligative Properties Carbon Tetrachloride Problem
« Reply #14 on: April 12, 2009, 07:04:16 PM »

Dear carp14;

Of course!,  -  slashes stands for division and mx is the "sign" for masses.

Good Luck!
                    ARGOS++

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