# HElp me with chem!

<p>How do you do this problem?</p>

<p>What is the minimum number of moles of PbSO4 that must be used to prepare 1 liter of saturated PbSO4 solution at 25 C?(ksp at 25 C for PbSO4 = 1x10^8)
a) 1x10^-16 mol
b) 1x10^-8 mol
c) 1x10^-4 mol
d) 1x10^-2 mol
e) 1x10^-1 mol</p>

<p>Also, are the tests in the PR book easier than the real test?</p>

<p>I think you meant to say that the Ksp for PbSO4 is 1*10^-8, not to the positive 8.</p>

<p>Anyway, it's just a matter of setting up an equilibrium. You'll only need to use the products though (Pb(2+) and SO4(2-)) since the only reactant, lead (II) sulfate is solid (this is the way solubility problems work).</p>

<p>So you get Ksp=[Pb++][SO4(2-)]=1*10^-8. Since there is one mol of Pb and one mol SO4 produced per mole of PbSO4, you can write this as:</p>

<p>Ksp=(x)(x), in other words Ksp=x^2. So 1<em>10^-8=x^2. Solve for x and you get 1</em>10^-4. This is called the molar solubility of PbSO4 (in mol/L). So the answer is C.</p>

<p>The tests in PR are about as hard as the actual test.</p>

<p>do you mean that the ksp is 1x10^-8? cause in that case:</p>

<p>ksp = [Pb2+][(SO4)2-]</p>

<p>Since they're going to be the same, substitute both for x</p>

<p>1x10^-8 = [x][x]
1x10^-4=x </p>

<p>so my guess is c</p>

<p>hahaha we both wrote like the same exact thing. good to know i got it right :)</p>

<p>In which process which the entropy increase? Alcohol boiling or the rusting of iron?</p>

<p>alcohol boiling</p>

<p>why does the rusting of iron not have an increase in entropy then ?</p>

<p>Entropy only increases when there's a conversion from solid to liquid or liquid to gas or solid to gas, and when the # of gas moles total increases from the reactant side to the product side.</p>

<p>Hey, does anyone know how hard exactly the SAT chem test is? I'm planning on taking it in June, but I'm only in Honors Chem. Do I have a chance of breaking 700 if I self study, or is all the material on the test AP chem stuff??</p>

<p>I took the SAT II chem after only honors chem as well. I used PR to prepare and got a 760. However, AP chem would have made it much easier... I'm positive I would've gotten an 800 had I taken it after AP Chem.</p>

<p>Thanks for the reply! That gives me hope, because I'm using PR too.</p>

<p>I feel like the PR book is too general sometimes. But then Barrons is too out there.</p>

<p>Goldshadow, did you use the PR AP book or the PR SAT2 book?</p>

<p>For the SAT II I used the PR SAT II book.</p>