|By J_Jay (J_Jay) on Saturday, May 10, 2003 - 01:39 pm: Edit|
Does any1 know a good Ap chem site that has real part1/part2's? i know about collegeboard.com, but is their nething else? thnx
|By Jason817 (Jason817) on Saturday, May 10, 2003 - 02:19 pm: Edit|
|By J_Jay (J_Jay) on Saturday, May 10, 2003 - 03:19 pm: Edit|
damnn those are really good thnx a lot
|By New (New) on Saturday, May 10, 2003 - 06:54 pm: Edit|
can anyone answer the following questions from 1999 mc? i don't get them:
28) The melting point of MgO is higher than that of NaF. Explanations for this observatoin include which of the following?
I. Mg2+ is more positively charged than Na+.
II. O2- is more negatively charged than F-.
III> The O2- ion is smaller than the F- ion.
a) II only.
b) I and II only.
c) I and III only.
d) II and III only.
e) I, II, and III
72. After completing an experiment to determine gravimetrically the percentage ofw ater in a hydrate, a student reported a value of 38 percent. The correct value for the percentage of water in the hydrate is 51 percent. Which of the following is the most likely explanation for this difference?
a) Strong initial heating caused some of the hydrate sample to spatter out of the crucible.
b) The dehydrated sample absorbed moisture after heating.
c) The amount of the hydrate sample used was too small.
d) The crucible was not heated to constant mass before use.
e) Excess heating caused the dehydrated sample to decompose.
75. Which of the following pairs of liquids forms the solution that is most ideal (most closely follows Raoult's law)?
a) C8H18 (l) and H20 (l)
b) CH3CH2CH2OH (l) and H20(l)
c) CH3CH2CH2OH (l) and C8H18 (l)
d) C6H12(l) and C8H18 (l)
e) H2SO4(l) and H20(l)
btw, how can i study for the color of precipitate/ solutions? any help is greatly appreciated.
|By J_Jay (J_Jay) on Saturday, May 10, 2003 - 06:58 pm: Edit|
uh i think for 28 the answer is b becuz well 3 isnt correct cuz individual atomic radii wouldnt affect the bond strengths...but the Mg0 has two +/- charges each, while NaF only has 1 each. Therefore Mg0 has stronger bonds 2 each other cuz its double, while NaF only has single makin it weaker. its tought to explain and im not sure if my reasoning is totalyl right, but it makes sense in my head
|By Sosodef (Sosodef) on Saturday, May 10, 2003 - 06:59 pm: Edit|
is there a website that has the 1999 released exam?
|By Jason817 (Jason817) on Saturday, May 10, 2003 - 07:45 pm: Edit|
28. Is it E?
They should all work because of Coloumbs Law. Either that or its B.
72. A? No idea.
75. No idea. Don't even know what's Raoult's law.
As for the colors, I get screwed up on those too. I don't know everything about it but I do know that all transition elements are not colorless except Zn 2+ because that has a filled 3d orbital.
|By Sixtoes (Sixtoes) on Saturday, May 10, 2003 - 07:59 pm: Edit|
28. i took this in class so i know /for sure/ that the answer is B. Coulombs law is E= k(Q+)(Q-)/d^2 or basically dependent on both distance between ions and individual ion charges. but individual ionic radii doesn't really matter. there was a better explanation, ill call a friend later and ask. edit: okay, "d" is the distance between the centers of the two particles, so size wouldn't make a difference. just for review, Coulomb's Law calculates the magnitude of the electronic force between two charged particles, aka the lattice energy or energy req. to separate a mole of solid ionic cpd into gas ions.
72. ok the % we are calculating is [mass of hydrate before heating] - [mass of anhydrous salt after heating] divided by [mass of hydrate before heating]. the experimentally determined percentage is smaller, meaning the [before]-[after] was too small, implying that the [mass after heating] is larger than the theoretical value. basically that means that the hydrate still has some water in it, so lets look at the options. B is the answer.
75. OK, Raoult's law as defined by my chem books is "A law stating that the partial pressure of a solvent over a solution, Pa, is given by the vapor pressure of the pure solvent, P*a, times the mole fraction of the solvent in the solution." You'll find more explanation on this in Vapor Pressure Lowering (colligative pressure) notes/book. Furthermore, "Real solutions best approximate ideal behavior when the solute concentration is low and when /the solute and solvent have similar molecular sizes and similar types of intermolecular attractions./ so if you look at the answer choices and notice similarities on very very basic terms, you'll see that D is the correct answer because both have only C and H and have similar shapes, attractions etc.
Um, for colors, I'd read any chapters in your book that may have not been assigned, just to skim through and notice if colors are mentioned. ALso, go through your old labs or the experimental section in your review book.
Any more questions? This is actually helping me review more!
|By Adamg (Adamg) on Saturday, May 10, 2003 - 10:32 pm: Edit|
28 Is B. You just have to figure out the lattice energy. III is incorrect right ooff the bat because an oxygen ion is bigger than a fluorine ion. Sixtoes explains the other two great also.
|By Adamg (Adamg) on Saturday, May 10, 2003 - 11:01 pm: Edit|
Does anyone know if studying what hybridization correlates with which molecule shapes would be a waste of time? I've taken some old tests and that is on there, but not on the 1999 test. Have they gotten rid of it?
|By Sixtoes (Sixtoes) on Saturday, May 10, 2003 - 11:32 pm: Edit|
Haha, yeah, i wasn't thinking about the oxygen ion being bigger than a fluorine ion. (For review in case anyone is wondering, the fluorine ion has more protons but is in the same energy "n" level as oxygen, since it has more protons, the effective nuclear charge "Zeff" is larger (protons - core electrons) meaning the valence electrons are more tightly held to the nucleus, so the ion is smaller)
as for hybridization, it may have not been on the '99 test but its a very popular question on both the AP and SAT-II in multiple choice, and its on the free response sometimes. just because something's not on the 1999 test doesn't mean it was taken off. study it!
|By Jason817 (Jason817) on Sunday, May 11, 2003 - 12:22 am: Edit|
I wouldnt go over hybridization. That's one of the toughest chapters and it rarely appears. But you never know...
|By Keyboardgunk (Keyboardgunk) on Sunday, May 11, 2003 - 01:43 am: Edit|
What does it take to get a 5 on this test?
|By Keyboardgunk (Keyboardgunk) on Sunday, May 11, 2003 - 01:46 am: Edit|
Where can I get my fingers on some old AP Chem Tests?
|By New (New) on Sunday, May 11, 2003 - 01:59 am: Edit|
hmm what's this commotion on spectrophotometer all bout?
what can they ask about spectrophotometers?
|By Jason817 (Jason817) on Sunday, May 11, 2003 - 03:00 am: Edit|
Keyboardgunk: Read the 2nd post in this topic
New: No idea. I don't even know what this is but I just heard that it is a likely free response question for the lab section.
|By Sixtoes (Sixtoes) on Sunday, May 11, 2003 - 01:48 pm: Edit|
the spectro thing is like a light you shine through a vial to determine molarity. you need to make sure the vial is full and that you reset the spectrometer or whatever by pointing it at a clear vial sometimes. i really have no idea though.
|By Adamg (Adamg) on Sunday, May 11, 2003 - 02:27 pm: Edit|
This is off topic but did anyone take the National Chemistry Olympiad Test?
|By New (New) on Sunday, May 11, 2003 - 02:46 pm: Edit|
let's keep it going
46) Which of the following occurs when excess concentrated NH3 is mixed thoroughly with 0.1 M Cu(NO3)2?
a) A dark red precipitate forms and settles out.
b) Seperate layers of immiscible liquids form with a blue layer on top.
c) The color of the solution turns from light blue to dark blue.
D) Bubbles of ammonia gas form.
e) The pH of the solution decreases.
65) Which of the following compounds is NOT appreciably soluble in water but is soluble in dilute hydrochloric acid?
42) When dilute nitric acid was added to a solution of one of the following chemicals, a gas was evolved. This gas turned a drop of limwater, Ca(OH)2, cloudy, due to the formation of a white precipitate. The chemical was
a) household ammonia, NH3
b) baking soda, NaHCO3
c) table salt, NaCl
d) epson salts, MgSO4 . 7H2O
e) bleach, 5% NaOCl
Step 1) N2H2O2 H2HO2- + H+ (fast equilibrium)
Step 2) N2HO20 -> N2O + OH- (slow)
Step 3) H+ + OH- -> H2O (fast)
Nitramide, N2H2O2, decomposes slowly in aqueous solutoin. This decomposition is believed to occur according to the reaction mechanism above. The rate law for the decomposition of nitramide that is consistent with this mechanism is given by which of the following?
A) Rate = k[N2H2O2]
B) Rate = k[N2H2O2][H+]
C) Rate = (k[N2H2O2])/[H+]
D) Rate = (K[N2H2O2])/[N2HO2-]
E) Rate = k [N2H2O2][OH-]
|By New (New) on Sunday, May 11, 2003 - 03:11 pm: Edit|
28) Which of the following is probably true for a solid solute with a highly endothermic heat of solution when dissolved in water?
a) The solid has a low lattice energy.
b) As teh solute dissolved, the temperature of the solution increases.
c) The resulting solution is ideal.
d) The solid is more soluble at higher temperatures.
e) The solid has a high energy of hydration.
69) A white solid is observed to be insoluble in water, insoluble in excess ammonia solution, and soluble in dilute HCl. Which of the following compounds could be solid be?
75) If a copper sample containing some zinc impurity si to be purified by electrolysis, the anode and the cathode must be which of the following?
Anode | Cathode
a) Pure Copper | Pure Zinc
b) Pure zinc | Pure copper
c) Pure copper | Impure copper sample
d) Impure copper sample | Pure copper
e) Impure copper sample | Pure zinc
|By Heatwave345 (Heatwave345) on Sunday, May 11, 2003 - 03:22 pm: Edit|
This is off topic but did anyone take the National Chemistry Olympiad Test?
I took it. got around 50/70. The questions were hard, but i had tons of stuff on my calculator.
46)C.Whenever i see the word excess, complex ions come to mind. the Cu(NO3)2 dissolves into Cu2+ ions (light blue color). Then EXCESS NH3 is added to make the Copperamine complex Cu((NH3)4)2+ (which is dark blue.
65) i would guess (A).B,C,D,and E are out because they are soluble in water.
|By Jason817 (Jason817) on Sunday, May 11, 2003 - 03:30 pm: Edit|
42. B because baking soda is white. That's the only reasoning I gave it.
|By Sixtoes (Sixtoes) on Sunday, May 11, 2003 - 03:38 pm: Edit|
46. the words "excess" and "concentrated" let you know that NH3 is acting as a complexing agent. Cu and NH3 don't precipitate, since NH3 isn't an ion. They don't form separate layers because they /complex/ meaning they mix thoroughly. C is correct, Cu to begin with is light blue and the complxing makes it darker. D makes absolutely no sense. E also makes no sense, if anything adding a weak base would increase the pH.
65. Check out your solubility rules first. B, C, D, and E are all soluble because NH4+ and NO3- are always soluble, and SO4(2-) is always soluble unless combined with strontium, barium, mercury-I, or lead ion. the only answer left is MgOH, which makes sense since its a weak base and would dissolve in acid.
42. we're looking for something that precipitates with OH-. you can cross off B, C, and E because it contains Na, which is always soluble. I'm stuck here- help anyone?
82. Clean up the question.
|By Sixtoes (Sixtoes) on Sunday, May 11, 2003 - 03:46 pm: Edit|
28. is D becase at higher temp's you have more energy, which is needed to dissolve.
69. It can be A or D or E after crossing out soluble choices. D or E after realizing that Ag and Zn form complex ions with NH3. And I think it would be E because its a weak base.
75. Far too lazy to do this but basically only A and B are possible, duhh it has to be pure. E*cell is [Ered + Eoxid] or more conveniently [Ered,cathode] - [Ered, anode]. whichever combo results in a positive E*cell answer is correct.
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