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AP Chemistry 2011 Study Thread

karaokemachinekaraokemachine Posts: 910Registered User Member
edited April 2011 in AP Tests Preparation
It's that time guys...

I've been doing some released exams for practice; the following are from the 2008 audit or official practice test from CB*. Can anyone explain them?

46. A closed rigid container contains distilled water and N2 (g) at equilibrium. Actions that would increase the concentration of N2 (g) in the water include which of the following?
I. Shaking the container vigorously
II. Raising the temperature of the water
III. Injecting more N2 (g) into the container

(A) I only
(b) II only
(C) III only
(D) I and II only
(E) I, II, and III

Answer: (C) III only


71. When a solution is formed by adding some methanol, CH3OH, to water, processes that are endothermic include which of the following?
I. Methanol molecules move water molecules apart as the methanol goes into solution.
II. Water molecules move methanol molecules apart as the methanol goes into solution.
III. Intermolecular attractions form between molecules of water and methanol as the methanol goes into solution.

(A) I only
(B) III only
(C) I and II only
(D) II and III only
(E) I, II, and III

Answer: (C) I and II only

*I would link the test but I'm not sure if I'm allowed...

Feel free to post other questions. I'll *attempt* to answer them.
Post edited by karaokemachine on
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Replies to: AP Chemistry 2011 Study Thread

  • karaokemachinekaraokemachine Posts: 910Registered User Member
    74. XY2 (aq) -> <- X2+ (aq) + 2 Y- (aq)
    A soluble compound XY2 dissociates in water according to the equation above. In a 0.050 molal solution of the compound, the XY2 (aq) species is 40.0 percent dissociated. In the solution, the number of moles of particles of solute per 1.0 kg of water is closest to

    (A) 0.15
    (B) 0.090
    (C) 0.070
    (D) 0.040
    (E) 0.020

    Answer: (B) 0.090 = .05mol x 3mol x 60%
    I figured out the calculation but I don't know the explanation.
  • Dogod11Dogod11 Posts: 394Registered User Junior Member
    For 46, you have to think what would result in the increase of concentration of N2 (g). Shaking the container would result in N2 (g) dissolving into N2 (aq) (because gasses dissolve when shaken), decreasing the concentration of N2 (g). Raising the temperature would shift the equilibrium to the endothermic side, which is increasing the amount dissolved, so would also result in a decrease in N2 (g).
    Adding more N2 (g) to the container increases the concentration of N2 (g) for reasons that I hope are obvious.

    For 71, water molecules and menthol molecules move each other apart, since that's what happens when menthol dissolves in water. This is an endothermic process, so I and II are correct. III is not correct because IMFs forming doesn't require energy, in fact it releases (a small amount of) energy.

    EDIT: For 74, are you sure XY2 is aq?
  • smugfiendsmugfiend Posts: 99Registered User Junior Member
    Shaking the container would result in N2 (g) dissolving into N2 (aq) (because gasses dissolve when shaken), decreasing the concentration of N2 (g).

    the question is asking what would increase the "concentration of N2 (g) in the water." your explanation would explain why choice I is correct, while it isn't. or am i reading the question wrong?

    anyway, the equilibrium concentration doesn't depend on how hard you shake the container because after the components of the container settle back down, the same equilibrium will be reestablished. temperature does affect the equilibrium position, but raising the temperature makes gases LESS soluble, so that's not what we're looking for. the third choice shifts the equilibrium to more N2 dissolved...by La Chatlier's principle or whatever, when you add a gas to the system, the water will attempt to dissolve the added gas to cancel out the added component.

    edit: i just typed my response to 71 and it deleted. ****.
    anyway, it's just stoichiometry. the molality is 0.05 m (meaning 0.05 moles of solute / kg solvent). the volume of water (the solvent) is 1 kg, so multiply them together and you get 0.05 moles of solute, that is, 0.05 moles XY2.

    there are two components to the calculation:

    1. 40% of XY2 dissolves into the three things on the right side of the reaction. (1 + 2 = 3)
    so 40% of 0.05 moles XY2, then times 3 = .....

    2. 60% of XY2 stays as it is. because XY2 is aqueous, it counts toward the number of dissolved particles.
    so 60% of 0.05= ....

    add up your answers for 1 and 2 and you get 0.09
  • karaokemachinekaraokemachine Posts: 910Registered User Member
    Thank you!

    @Dogod11 How is dissolving menthol in water an endothermic process?
    And yes, I'm sure it's XY2 (aq).
  • theeboy3theeboy3 Posts: 191- Junior Member
    71. The question is asking which of the following three processes are ENDOTHERMIC.

    #1 is endothermic because energy is required to separate the water molecules. In order for the two substances to mix (they do, both are polar), energy must be added to break the water - water bonds and the methanol - methanol bonds.

    #2 is endothermic for the same reason. Energy is inputted to break methanol-methanol bonds.

    #3 is not endothermic, as the bonds reforming release energy.
  • karaokemachinekaraokemachine Posts: 910Registered User Member
    Merci beaucoup!
  • karaokemachinekaraokemachine Posts: 910Registered User Member
    Here's the random info I've compiled from released exams, in case anyone's interested. Surprisingly, only some of them have random facts.

    1994
    11. Utilized as a coating to protect Fe from corrosion: Zn
    12. Is added to silicon to enhance its properties as a semiconductor: As
    13. Utilized as a shield from sources of radiation: Pb
    2008 Audit
    1. Commonly used as a disinfectant for minor skin wounds: H2O2
    2. A refrigerant implicated in the thinning of the stratospheric ozone layer: CCl3F
    3. A major component of the fuel known as natural gas: CH4
  • smugfiendsmugfiend Posts: 99Registered User Junior Member
    thanks for the list. personally i wouldn't spend too much time studying these exceptions. if you know the general general general information well, you'll get a 5.

    the one about the Zn protecting Fe is in the PR book for electrochem...which i just did today in the back of class. :D

    what's 2008 audit?
  • karaokemachinekaraokemachine Posts: 910Registered User Member
    @smugfiend The 2008 audit is the official practice test that's released by CB to AP teachers.
  • theeboy3theeboy3 Posts: 191- Junior Member
    What are you guys doing now in class?
  • GreedIsGoodGreedIsGood Posts: 2,039Registered User Senior Member
    can we go over the question about calculating heat of fusion for water?
  • GreedIsGoodGreedIsGood Posts: 2,039Registered User Senior Member
    Also, if you want to measure 100mL of a solution do you use:
    (A) 50mL pipet
    (B) 100mL erlenmyer
    (C) 100mL volumetric
    (D) 100mL graduated cyclinder
  • smugfiendsmugfiend Posts: 99Registered User Junior Member
    What are you guys doing now in class?
    for my class, nearly done with electrochem and only nuclear chem remaining i think.
    can we go over the question about calculating heat of fusion for water?
    where is that? also, i don't know about your second question. in terms of practicality, all of them would theoretically work... no idea
  • karaokemachinekaraokemachine Posts: 910Registered User Member
    We're doing released exams for HW and reviewing them in class. We have to do nuclear chem on our own.

    @GreedIsGood What's the question about the heat of fusion for H2O?
    To measure a 100mL solution, use a (C) 100mL volumetric cylinder because it's the most accurate.
    (A) Pipet is useless
    (B) Erlenmeyer is the triangular-shaped one with the short neck
    (C) Volumetric is the triangular-shaped one with the long skinny neck with only 1 line (for 100mL)
    (D) Graduated cylinder is simply the cylinder one
  • smugfiendsmugfiend Posts: 99Registered User Junior Member
    oh...i was thinking about the high-tech pipets that let you turn the dial and stuff lol.

    karaokemachine or anyone else, do you have any of the released exams i could possibly share?
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