A few chem questions from The Official CB Subject Test book

Test Preparation SAT Subject Test Preparation
1

<p>In Collegeboards Official SAT Subject Tests, I have a few questions about its Chemistry Test.</p>

<h1>4 --> Observed when 1.0 mol of Na2CrO4 and 2.0 mol of Pb(NO3)2 are mixed with 1 L of water.</h1>

<p>I chose E) No solid and colorless solution.
BUT, the answer is A) Yellow solid and colorless solution. (CB explicitly says that Na2CrO4 is "a soluble yellow solid" and Pb(NO3)2 is "a soluble white solid".</p>

<h1>5--> Observed when 3.0 mol of Na2CrO4 and 1.0 mol of Pb(NO3)2 are mixed with 1 L of water.</h1>

<p>I chose D) No solid and yellow solution.
BUT, the answer is B) Yellow solid and yellow solution.</p>

<h1>115--></h1>

<p>Statement I: A fluoride ion, F-, and an oxide ion, O2- have the same diameter.</p>

<p>Statement II: The fluoride ion, F-, and the oxide ion, O2-, have the same number of electrons.</p>

<p>I put T T CE.
But the answer is F T. Why? Wouldnt both ions have the same number of electrons, therefore the same size of electron clouds so the same diameter as well?</p>

<h1>44-->Which of the following statements is true concerning a saturated solution of a salt at a constant temperature.</h1>

<p>A) The concentrations of salt and solvent are usually equal.
B) The amount of dissolved salt is constant.
C) Addition of solid salt shifts the equilibrium, which results in an increase in the amount of dissolved salt.
D) The solution is unstable and sudden crystallization could occur.
E) At the same temperature, a saturated solution of any other salt has the same concentration.</p>

<p>I was torn between B and C...but B is the answer. Arent both conditions valid in a saturated solution?</p>

2

<p>How about you write the question?</p>

<p>I don't feel like checking my book lol</p>

3

<p>eye tuk duh peesat, n on dere dis iz how eye dihd it:</p>

<h1>4: H2O is water mayn...helo?</h1>

<h1>5: dat dun even make cents mayn...dat dun even make cents.</h1>

<h1>115: ew be ovathinkin stuff mayn...OXYGEN n FLUORINE be diffren mayn...</h1>

<h1>44: mayn D be bout a SUPASATCHRATED salootion...N E dun even mayk cents...wut u tinkin C fo? B be duh ansa mayn...B be duh ansa</h1>

4

<p>Hey Calculus,</p>

<p>Those basically are the questions. :) The first two are those list-type questions, and the last two are exact questions.</p>

<p>Thanks!</p>

5

<p>I don't own CB Subject Test Book, but anyways:</p>

<h1>115 even if they both have same number of electrons, they have different number of protons, thus changing the size of F- and O2-.</h1>

<h1>44 in option (c), no more salt would dissolve, because it is already saturated.</h1>

6

<p>Ah, Deftech, thanks so much I didn't even think of the nuclei of the ions.</p>

<p>For #44, wouldn't the system just find equilibrium again after the concentration disturbance?</p>

7

<p>For 44, it can't be C because the solution is saturated. Thus, the amount of dissolved salt CANNOT increase.</p>

8

<p>I believe #4 and #5 deals with insoluble solid and that's why they precipitate.
I don't know exact color though; that would have to deal with lab.</p>

<h1>44. B is the answer because no more salt will dissolve; and thus the amount of dissolved salt is constant.</h1>

<p>Why the salt will no longer dissolve, the solution is already saturated and the temperature is constant.</p>

<h1>115 Diameter would imply atomic radius and I believe O2- has a bigger atomic radius than F-.</h1>

9

<h1>4: When you mix 1 mol of Na2CrO4 and 2 mols Pb(NO3)2, you get PbCrO4 and NaNO3, along with some leftover Pb (II) ions. PbCrO4 is an insoluble yellow solid. NaNO3 is soluble however, so it does not precipitate. Therefore, you're left with a colorless solution with yellow PbCrO4.</h1>

<h1>5: Again, you end up with yellow precipitate because of the insoluble PbCrO4. However, this time you have an excess of chromate ions which will float around in solution. These give the solution a yellow color.</h1>

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<p>dude listen to peesat.</p>

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<p>for #44, if you added more solute to a saturated solution, wouldn't the solution then become supersaturated the moment you add more solute and then immediatley crystallize? But that's not an answer option so I don't know. I'm getting confused with saturation/supersaturation/unsaturation connected to equilibrium. Anyone able to explain please? :)</p>

12

<p>Supersaturation only occurs when a solution's conditions, ie temperature or pressure, are changed. That is not the case here.</p>

<p>If you add solute to an already saturated solution, then it does not increase the amount dissolved. That's because there's no net dissolution or solvation, because solute crystallizes out at the same rate that it dissolves(it's in equilibrium).</p>

<p>I think you should go back and review most of these concepts in whatever prep books/textbooks you have, because you should know them very well for the SAT II.</p>