[CHM 115] Lab

[Andrew J. Pounds]

On 10/26/14 15:19,  wrote:
> Hi Dr. Pounds!
> I'm trying to do the lab, and we either really messed up in lab or I'm 
> calculating the deltaH values for reactions three and four wrong. I 
> think my mistake might be in calculating the mass of the solution for 
> these reactions. Could you give me any pointers for calculating the 
> mass of the solution for reactions three and four?
> Thanks!
>

So, in the equation $q = m c_s \Delta T$ you need to use the mass of the 
solution.  You know the volume and the density of the solution is listed 
on the lab report form.  You can therefore use $m = \rho V$ to compute 
the mass of the solutions.   The specific heats for these solutions can 
also be found on the report form.

Now, when it comes to $\Delta H$, you have to remember that it is the 
constant volume heat capacity on a per mole basis.   So take your $q$ 
that you determined and divide it by the moles of substance.

In the case of the dissolution of NaOH you have roughly 2 grams of 
NaOH.  Convert this to moles  (2 g / 40 g/mol) = 0.05.  So your enthalpy 
of reaction for the dissolution of NaOH should be $q / 0.05$.

In the case of the reaction of NaOH (aq) with HCl (aq) you have 50 ml of 
1 M NaOH (that would also be 0.05 moles), so divide the heat by that 
amount to get the enthalpy like you did before.

Hopefully that give you enought to get through the rest of the lab, but 
let me know if you have any questions.





-- 
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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