<p>Do you think the AP Chem sample questions are the level that will be on the AP? if so… ouch, they’re so much harder than the practice i was doing in princeton review
That said, i have a bunch of questions</p>
<li><p>2SO2 + O2 → 2SO3
A mixture of gases w. .2 mol SO2 and .2 mol O2 in a 4L flask reacts to form SO3, if temp is 25 C, whats the pressure in the flask once the rxn is complete? (Answer is (.3 x .082 x 298)/4 …where did we get the .082 from?)</p></li>
<li><p>Which of the following species is reduced? Answer is NO3- → NO… but it loses an electron, and is therefore oxidized, not reduced, no?</p></li>
<li><p>A sample of CaCO3 (molec mass 100g) was reported as being 30% Ca. Assuming no Ca was present in any impurities, the % of CaCO3 in the sample is… (Answer is 75%, I have no idea how they got that)</p></li>
<li><p>Maybe I’m a ■■■■■■ on this one, but i cant find my mistake
__ Cr2O7 2- + _<em>HNO2 + _</em><em>H+ → _</em><em>Cr3+ + _</em><em>NO3 - + _</em>_H2O
I balanced it as
Cr2O7 2- + 2HNO2 + 8H+ → 2Cr3+ + 2NO3 - + 5H2O, but apparently theres supposed to be a 4 in front of H2O</p></li>
<li><p>Which of the following salts forms a basic solution in water NaCl (obviously not), (NH4)2SO4 (acidic), CuSO4 (why not? SO4 is a conjugate base), K2CO3 (the correct answer, apparently), NH4NO3 (acidic)</p></li>
</ol>
<p>(And if anyone has the questions in front of them, 29 also, but its a bit much to type)</p>
<p>Thanks!</p>
<p>11) 0.082 is R in PV=nRT
14) Don't look at the charge, look at individual oxidation numbers. In N in NO3- is +5, N in NO is +2, thus reduction.
16) CaCO3 is 40% calcium. therefore if the sample is only 30%calcium, it must only be 3/4 calcium carbonate
18) I'll check and get back to you
27) Poor question. conjugates of strong acids are not acidic, but SO4 is the conjugate of HSO4 which is not very strong. Therefore, SO4 will be a weak base.
29) Don't have the questions, sorry</p>
<p>These questions are typical of the MC</p>
<p>18)</p>
<p>6e + 14H+ + Cr2O72- --> 2Cr3+ + 7H2O</p>
<p>(H2O + HNO2 --> NO3- + 3H+ + 2e) x 3</p>
<p>When combined, 4H2O's remain on the right.</p>
<p>How is 27 a poor question? I know you can't tell if it will be basic or acidic for (NH4)2 SO4 unless you get the Ka for NH4+ and Kb of SO4-. If the Ka of NH4+ is more than 2 times the Kb of SO4-, then it is acidic solution.</p>
<p>I was looking more at the CuSO4, but for both sulfates, there's insufficient information to tell if it's acidic or basic. As you pointed out, when both ions have acid/base properties, you have to compare K's. K2CO3 is clearly basic, but the others may be also. I just thought it was a little ambiguous. (And usually the MC's don't have much, if any, ambiguity.)</p>
<p>for #16. how did you know its 40% ca?
and #11 why is it .3 for moles?</p>
<p>Ca's MW is 40.1 g/mol or about 40. Since CaCO3 is 100 g/mol, Ca is 40% of CaCO3 by weight. The actual sample had only 30% Ca so it must have been 30/40=3/4-75%</p>
<p>SO2 is the limiting reagent so all 0.2 moles of it gets used up. Using stoichiometry, 0.1 moles of O2 gets used up, leaving 0.1 moles of O2.</p>
<p>Using stoichiometry again, 0.02 moles of SO3 forms.</p>
<p>Add these 2 bolded numbers to get 0.3 moles of gas.</p>
<p>SO4 is the conjugate of H2SO4, which is a strong acid.
CO3-2 is the conugate of H2CO3, a weak acid.</p>
<p>CO3-2 is a weak base.</p>