SAT Chem Help

<p>Hi, I'm taking the test in 2 days and I've been cramming like crazy,
I took the official practice test today (big blue book) and I have one main question, #65.
It gives you an experiment about collecting H gas using HCl and Mg metal. It gives you data (atmospheric pressure=749.8mm Hg, vapor pressure of H2O=19.8mm Hg, volume of H gas collected over water=25.2ml, water/room temp=22 C, mass of Mg=.024g)
Reaction is Mg(s) + 2H(+) --> Mg(2+) + H2(g).<br>
The question asks: "The volume of the dry hydrogen gas at 1 atm and room temperature would be"
Correct answer is C, ((25.2)(749.8-19.8)) / 760 mL. Could someone explain this to me? </p>

<p>2nd question:
Why does mixing 1L He gas and 1L Ne gas in a 2L flask increase entropy?</p>

<p>In the first section there is also a set asking about precipitates from mixing Na2CrO4, PbCrO4, NaNO3, Pb(NO3)2 (given that they are solids, soluble/insoluble, color) and it asks for results when 2 of them are mixed in water. Would the color of the solution be determined by if there is excess of one solid after precipitation?</p>

<p>Lastly, how much does the test ask about "common" chemicals (like what chemical is in household bleach)?</p>

<p>Thanks.</p>

<p>(Also unrelated question, how hard is the US history SAT? I went through Barrons taking notes on what seemed important and skimmed through PR but my practice tests haven't been great. They seem to ask about authors and books as well as quotations and who said them. Any last minute tips or vital information I should know?)</p>

<p>1st question: The partial pressure of hydrogen gas P(H2) = P(atm) - P(vap)= 730 mmHg
By combining the laws of Boyle, Charles and Gay-Lussac, we derive the equation
P1V1/T1 = P2V2/T2
Because T1 and T2 are both room temperature, they cancel out in the equation and therefore, V2 (the volume you need to calculate) = P1V1/P2 = (730mmHg*25.2mL)/760mmHg
Note that P2 = 760 mmHg because H2 is dry and 1atm = 760 mmHg.</p>

<p>I'm afraid I'm not quite sure what you're asking on your other questions.
The test doesn't ask much about common chemicals and even if it does, it usually provides its chemical formula.</p>

<p>To answer your second question, entropy is basically a measure of how disordered the system is. So since you're mixing two gases, you're creating more "disorder" in the system. If that makes any sense?</p>

<p>I'm not really sure about the third question. If you're asking if the color is determined by whichever ion is in excess in solution, then I think you're correct. All of the other ion (or most/to a considerable degree) would be precipitated out if the first is in excess, leaving only ions of the first type in solution.</p>

<p>i agree with the first two responses, for the third question, the remaining color is determined by what ions are left in solution, so you are correct.</p>

<p>It depends on how fluent you are in History. For me, the SAT subject for history is my worst subject.</p>