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<div class="moz-cite-prefix">On 02/05/2018 04:04 PM,wrote:<br>
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<p style="margin-top:0;margin-bottom:0">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! </p>
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<p>Great question... If you remember back to the thermo chapter
work was <img alt="$P\Delta V$" style="vertical-align: middle;"
src="cid:part1.883BC1EB.B46403A7@mercer.edu">. Now, if you
break down Pressure, it is the force per unit area.</p>
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<p><img alt="$P = \frac{\frac{kg m}{s^2}}{m^2}$"
style="vertical-align: middle;"
src="cid:part2.CD762079.18614C82@mercer.edu"></p>
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<p>the numerator above is kilograms time mass divided by seconds
squared, i.e. - <img alt="$F=ma$" style="vertical-align: middle;"
src="cid:part3.30E65F32.CF630E16@mercer.edu"> , and the
denominator is the area squared in meters.</p>
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<p>If we multiply that by volume, <img alt="$m^3$"
style="vertical-align: middle;"
src="cid:part4.85E2A786.7DD98BB2@mercer.edu">, we get</p>
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<p><img alt="$PV = \frac{\frac{kg m}{s^2}}{m^2} m^3 = \frac{kg
m^2}{s^2}$" style="vertical-align: middle;"
src="cid:part5.9B1E78AD.7C50E06F@mercer.edu"></p>
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<p>That last term on the right contains the derived units for a
JOULE. <br>
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<p>When you doing problems using the ideal gas equation, I recommend
using R in the units of <img alt="$\mathrm{\frac{L\cdot
atm}{^{\circ}K\cdot mol}}$" style="vertical-align: middle;"
src="cid:part6.5701098D.CD2665B9@mercer.edu">. 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 <img
alt="$\mathrm{\frac{J}{^{\circ}K\cdot mol}}$"
style="vertical-align: middle;"
src="cid:part7.7E83E632.E49BA1BA@mercer.edu">.</p>
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<pre class="moz-signature" cols="72">--
Andrew J. Pounds, Ph.D. (<a class="moz-txt-link-abbreviated" href="mailto:pounds_aj@mercer.edu">pounds_aj@mercer.edu</a>)
Professor of Chemistry and Computer Science
Mercer University, Macon, GA 31207 (478) 301-5627
<a class="moz-txt-link-freetext" href="http://faculty.mercer.edu/pounds_aj">http://faculty.mercer.edu/pounds_aj</a>
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