<p>@kian</p>
<p>That wasn't his question though. I don't understand either why their G is 0. Just take it as it is I guess.
G=H-TS
H=0
G=0-TS
G=TS</p>
<p>Can somebody explain when to use 25 degrees Celsius and when to use 0 degrees celsius?</p>
<p>0 is standard temp....25 is room temp...that's all.</p>
<p>Yeah, but sometimes they use 25 degrees celsius.</p>
<p>if your talking about gases and their volumes, chances are it will be 0 Celcius because that it Standard temperature for gases. BUT, if you're doing a thermodynamic problem, like one with delta H, delta S, or delta G, it's probably going to be at 25 because that is the standard THERMODYNAMIC temperature. Other than that, AP will NOT make you decide which value to use. they will always tell you what temp to work at.</p>
<p>For equilibrium concentrations in titrations, is this correct:</p>
<p>[OH]- = sqrt(Kb*(initial [OH-]</p>
<p>I'm not sure if this is right...</p>
<p>electrochemistry questions also use 25 celsius as the temp ex: Calculate the equilibrium constant at 25 degrees</p>
<p>What pmahesh is referring to is the equation for the correction of free energy at nonstandard conditions.</p>
<p>Delta G = Delta G (standard) + RTln(K)</p>
<p>Of course Delta G is 0 at equilibrium, so..</p>
<p>0 = Delta G (standard) + RTln(K)</p>
<p>How do you do this problem?
Which of the following substances is LEAST soluble in water?
A) (NH4)2SO4
B) KMnO4
C) BaCO3
D) Zn(NO3)2
E) Na3PO4</p>
<p>The answer is C, but how do you go about finding that answer?</p>
<p>eirrac89,</p>
<p>For those you have to know you solubility rules.</p>
<p>All alkali metal & ammonium ions are soluble, so you can eliminate A, B, and E.</p>
<p>All nitrates are soluble, so you can eliminate D.</p>
<p>And finally, all carbonates, except those attached to alkali metal & ammonium ions, are insoluble. So it has to be C.</p>
<p>you just gotta know your soluble ions lol</p>
<p>So deltaG naught is at 25 degrees celsius?</p>
<p>Also, can anybody please do #13 from Princeton review page 233? (2006-2007 edition).</p>
<p>It gives red. potentials for H20, Na+, and Cl2 to be -0.8,-2.7,+1.4 respectively. It says the electrodes are attached to terminals of a 2 V battery and the electrodes are in a solution of sodium chloride.</p>
<p>Which of the following form? (select all that are true)</p>
<ol>
<li>Chlorine gas at anode</li>
<li>Hydrogen gas at anode</li>
<li>Solid sodium at anode</li>
<li>Chlorine gas at cathode</li>
<li>Hydrogen gas at cathode</li>
<li>Solid sodium at cathode</li>
</ol>
<p>Goodness I loathe those problems.</p>
<p>Step one: arrange the half-reactions so that what you start out with (H2O, Na+, Cl-) are in the reactants. That may mean you have to reverse the half-rxn and therefore change the sign of the red. potential.</p>
<p>H2O --> whatever (I don't know the exact half rxn) -0.8
Na+ --> Na + e- -2.7
[this one you have to reverse] 2Cl- --> Cl2 + 2e- -1.4</p>
<p>I believe, the one with the most positive reduction potential reduces, and the one with the second most positive oxidizes.</p>
<p>Therefore, chlorine gas is formed at the anode, and hydrogen at the cathode.</p>
<p>That's right, but it's anti-common sense. The second most positive oxidizes is not common sense.</p>
<p>To clarify:</p>
<p>Use 0 degrees only in gas problems?</p>
<p>Also, what equation was it that you're not supposed to convert pressure into molarity? Was it Nernst?</p>
<p>Also, will there be questions on positron emissions and such? I took a past AP exam and it only had one problem on nuclear chem and it was simple with the equation already provided. (a nuclide collision blahblahblah).</p>
<p>On average, there is only one question on the multiple choice section on nuclear chemistry, and almost never on the free response.</p>
<p>I just did the 2006 FR part and on top of the 1st page it says "Attention should be given to significant figures", so if I round my answer incorrectly, will it be marked as "wrong"?</p>
<p>i think u may get a few points off but i think if its in the ballpark then u get no points off, they set the rule to a certain extent i think</p>
<p>What percent of FR and multiple choice do you think will be required for a 5 this year?</p>