Chemistry - Help

Pre-College Issues High School Life
1

<p>I'm sitting here at 3:00 am trying to finish this chemistry take home test (allowed to get help from anyone and the book), and some of the questions I'm unsure of. I know there are alot of good chem students on here so this should be easy. It's only gas laws. :D I know I can't ask you to do it for me, but I would appreciate tips/explanations of how to start some of these problems and maybe you could tell me if I'm wrong on some of them.</p>

<p>1. Theoretically, a gas would have zero volume at absolute zero. Why would it never actually achieve this volume?</p>

<p>Ok, I've searched endlessly on this one, as easy as it seems. I'm thinking it's because gas is always in motion? I dunno.</p>

<p>2. People sometimes play pranks involving the discharge of compressed carbon dioxide fire extinguishers. This action is very dangerous and should not be done because the CO2 can freeze skin and cause severe frostbite. Explain this danger, considering that the gas inside the extinguisher is at room temperature.</p>

<p>If it's at room temperature, how does it cause severe frostbite? I know it has something to do with the pressure inside the container and how it has to equalize when it is released from it. Any ideas?</p>

<p>3. If a student performs an experiment to determine the molecular mass of a gaseous compound using the modified ideal gas equation M=mRT/PV and forgets to correct for the fact that the gas was collected over water, would the results be high or low? Explain your answer.</p>

<p>Water would add to the pressure, or the volume I think. I guess it would make the results higher since the water vapor is added. Hints?</p>

<p>I've gotten all the mathematical problems right, but these conceptual problems are confusing to me.</p>

2

<ol>
<li><p>You're on the right track. It's basically just the third law of thermodynamics. If we were to reach absolute zero, entropy would have to be 0, which is impossible because the molecules are never going to stop completely moving. Even in super-cooled solids, they'll still be vibrating a little.</p></li>
<li><p>Not quite sure about this one... but,
(P<em>i)(n</em>f)(R<em>f)(T</em>f)/(V<em>f) = (n</em>i)(R<em>i)(T</em>i)(P<em>f)/(V</em>i)
Since, (n<em>f) = (n</em>i), (R<em>f) = (R</em>i)
(P<em>i)(T</em>f)/(V<em>f) = (P</em>f)(T<em>i)/(V</em>i)
(P<em>f) will be lower, (V</em>f) will be higher, so (T_f) will have to be lower.
So, if you suddenly change the pressure (1 atm)</p></li>
<li><p>If the gas was collected over water, you'd have to deal with Dalton's Law of Partial Pressures. I think if I remember right, you're going to obtain a higher experimental result than the theoretical value. So, yeah... you were right about this one too, I think.</p></li>
</ol>

<p>Hope this helps! It's been sort of a long time since I took general chem, so I might be a little rusty ;)</p>

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<p>Thanks! You're probably better at chem me. My teacher basically hands out a bunch of work and expects us to do it on our own. She doesn't teach, she just rushes through some bad powerpoints.</p>

<p>I knew #1 had to do with thermodynamics. It kept popping up when I was searching for absolute zero. We haven't covered it yet, but it's easy enough to explain.</p>