[CHM 112] lab report

[Andrew J. Pounds]

On 02/13/14 06:29, wrote:
>
> ​Dr. Pounds,
>
>

First -- the cooling curve should have TWO LINES on it;  one is for 
where the temperature drops rapidly and the other one for where it 
levels off.  The intersection of these two lines is the freezing point.  
If you are still confuses, please look back at the figure at the end of 
the lab procedures (which has one cooling curve on it) and then also at 
my sample plot on the class web page (which has all six cooling curves 
on it).  I am not requiring you to put all of the cooling curves on the 
same plot.

> I'm confused as to how I can find the moles of solute since it's an 
> unknown. I'm also having difficulties with my graphing. I can graph 
> all of the values but I can't seem to graph two lines on the same 
> graph in order to figure out where the two intersect. Thank you for 
> your help.
>
>
Lets call the freezing point of the pure solid $T_1$ and the freezing 
point of the solid with an unknown $T_2$. $\Delta T = T_1 - T_2$.

Also, $\Delta T = K_f m$ where $m$ is the molality.  The molality is 
moles of solute (unknown) over kg of solution.

Let's rearrange...

$\Delta T = K_f m$

$\Delta T = K_f \frac{n}{{\mathrm{kg\ solvent}}}$

$\frac{{(\Delta T)}{(\mathrm{kg\ solvent}})}{K_f} = n$

Since I now have $n$, the moles of unknown, and the mass of the unknown, 
I can determine its molar mass.


Does that help?


> Sincerely,
>
>
>
> -- 
> Andrew J. Pounds, Ph.D.  (pounds_aj at mercer.edu)
> Professor of Chemistry and Computer Science
> Mercer University,  Macon, GA 31207   (478) 301-5627
> http://faculty.mercer.edu/pounds_aj


-- 
Andrew J. Pounds, Ph.D.  (pounds_aj at mercer.edu)
Professor of Chemistry and Computer Science
Mercer University,  Macon, GA 31207   (478) 301-5627
http://faculty.mercer.edu/pounds_aj

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