[CHM 112] Quiz Help!

[Andrew J. Pounds]

On 02/05/2018 04:04 PM,wrote:
>
> Hey Dr Pounds, I have another more specific question. I know I asked
> in class but I need a bit more explanation because I am a bit confused
> on when to use the correct R value - 8.314 J/mol or .08206 L/mol.
> Thanks again! 
>
>
>
> ------------------------------------------------------------------------
>

Great question...  If you remember back to the thermo chapter work was
$P\Delta V$.  Now, if you break down Pressure,  it is the force per unit
area.


$P = \frac{\frac{kg m}{s^2}}{m^2}$


the numerator above is kilograms time mass divided by seconds squared,
i.e. - $F=ma$ , and the denominator is the area squared in meters.


If we multiply that by volume, $m^3$, we get


$PV = \frac{\frac{kg m}{s^2}}{m^2} m^3 = \frac{kg m^2}{s^2}$


That last term on the right contains the derived units for a JOULE. 


When you doing problems using the ideal gas equation, I recommend using
R in the units of $\mathrm{\frac{L\cdot atm}{^{\circ}K\cdot mol}}$. 
That way you can do all of your calculations you will be in the
"natural" units of the problem.   When you do calculations where you
have to take ratios with enthalpies (which are typically in kJ/mol) or
compute molecular speeds, then use the form in the units of
$\mathrm{\frac{J}{^{\circ}K\cdot mol}}$.




-- 
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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