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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">I got the following question:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">**********<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">“</span><span style="font-size:12.0pt;color:black">I am currently reviewing some chapter 15 concepts and am a little confused pertaining<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black">to when the volume matters in an equilibrium problem. I know that volume does not<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black">affect the K constant, but I remember for some problems done in this chapter, if the<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black">question was given and included a volume of 2L, we would have to divide each value<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black">by 2L to get the 1L value. <o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black">I may just be completely misinterpreting something, but for example I am on question<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black">15.33 of the EOC questions and the volume is said to be 5L. Why are the pressure values<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black">given not divided by 5L to obtain the 1L value?<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black">I guess in conclusion, when should we worry about the volume of a vessel, if ever? I do<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black">know shifting in a reaction is affected by volume and pressure.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">**********<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">The value of K doesn’t depend on the volume (although the molarities and pressures used<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">in K do depend on the volume of the container). So when is volume important?<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">If a problem gives you moles and a volume and asks you to solve for Kc or you’re given<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">Kc and asked to find the conc. of one of the substances in the expression for Kc you need<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">to convert the moles to molarities.
<b>You need to use molarities in the expression for Kc.<o:p></o:p></b></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">For a <b>Kp you should be using partial pressures in atm</b>. If you’re given masses or moles<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">of substances, temperature and the container volume you need to convert to pressure (in<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">atm) before substituting into the express for Kp.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">This is why when we do ICE table problems we convert to molarities or atmospheres and<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">use those in the ICE table. If you don’t use molarities in Kc or atm in Kp you aren’t going<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">to get the correct answers for the conc. or pressures at equilibrium.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">I know this can get a little confusing because in Ch 17 we need to use moles for neutralization<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">reactions when a strong acid or base is added to a buffer system or when doing a titration.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">A neutralization rxn involving a strong acid or base goes to completion and we’re dealing with<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">a limiting-reactant stoichiometry problem. These are done in moles using a BCA table (or as<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">I call them, ICC table). These tables look like ICE tables but are done in moles.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">For EOC exercise 15.33 it gives a rxn and partial pressures in atm and asks you to determine<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">Kp at 500.0 K in part (a). In part (b) it gives a volume of 5.0 L and asks for Kc. The volume<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">is a “red herring”. It’s not needed to answer the question. You should remember the<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">relationship between Kp and Kc,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"> Kp = Kc (RT)^delta(n)<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">There is no volume in this eqn. so you don’t need it. It’s one of those questions that really<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">gets at how confident you are with the material. You need to quickly recognize you don’t need<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">the volume to answer the question. This is like being given the density in a conc. conversion<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">problem when you don’t need it.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">Dr. Zellmer<o:p></o:p></span></p>
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