AP Chemistry exam question (multi-choice): What should I know?

In summary: So if you were to try and answer this question without knowing this information, you would be correct if you guessed c and e, but not if you guessed a or b.
  • #1
kotreny
46
0
This is a multiple choice question supplied by the AP Chemistry Course Description, available at apcentral-dot-collegeboard-dot-com. I'm not sure what you are supposed to know to answer it.

http://apcentral.collegeboard.com/apc/public/repository/ap-chemistry-course-description.pdf


Homework Statement



29. The table below summarizes the reactions of a certain unknown solution when
treated with bases.

Sample___Reagent____Limited amount of Reagent____Excess Reagent
I....NaOH(aq)...White precipitate.....Precipitate dissolves
II...NH3(aq)...White precipitate.....White precipitate

Which of the following metallic ions could be present in the unknown solution?
(a) Ca2+ (aq)
(b) Zn2+ (aq)
(c) Ni2+ (aq)
(d) Al3+ (aq)
(e) Ag+ (aq)

Homework Equations



The Attempt at a Solution



To be honest, I had no clue how to answer this question when I first saw it. The only thing helping me was the uncertain memory that Ca(OH)2 could precipitate if in excess. Admittedly, I peeked at the answer (d) and did some research. I now know that Al(OH)3(s) dissolves in both acidic and basic solutions (as Al3+ and Al(OH)4-, respectively), so it is consistent with the results from both samples; Al ions combine with hydroxide from NaOH, forming a precipitate of aluminum hydroxide which dissolves if too much hydroxide is added. Adding ammonia increases hydroxide ion concentration too, but the effect is much smaller so there's never a threat of excess. Zinc actually behaves similarly, but it turns out its hydroxide forms a soluble ammine complex.

My question is, do you really need to know all this--in my opinion, trivial--knowledge, or is there a better way? Must you learn specifically about individual metal hydroxides to answer this particular question?

I understand that at least some chemical trivia is useful for having a "chemical intuition", but this seems a bit much. You would either need to memorize the detailed properties of aluminum hydroxide, or know something about all the choices. Of course, on the exam, this could be one of several diverse questions about chemical trivia, with the expectation that only a fraction will be answered correctly by even the most knowledgeable students. But please tell me if I'm missing something here.
 
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  • #2
I am afraid you need to know all these facts about individual ions. Chemistry is a mix of things that you have to understand and things that you need to remember.

However, it is not as tough as it may look. Only two of the ions listed are amphoteric (so they dissolve in excess strong base), of these two one doesn't get complexed by ammonia.
 

Related to AP Chemistry exam question (multi-choice): What should I know?

Q1: What is the format of the AP Chemistry exam?

The AP Chemistry exam consists of two main sections: multiple-choice questions and free-response questions. The multiple-choice section contains 60 questions that must be completed in 90 minutes, while the free-response section contains 7 questions that must be completed in 105 minutes.

Q2: What topics are covered on the AP Chemistry exam?

The AP Chemistry exam covers a wide range of topics, including atomic structure, chemical bonding, thermodynamics, kinetics, equilibrium, acids and bases, and electrochemistry. It also includes laboratory skills and mathematical problem-solving.

Q3: How should I prepare for the AP Chemistry exam?

To prepare for the AP Chemistry exam, it is important to review all of the topics covered in the course and practice answering multiple-choice questions and free-response questions. It is also helpful to use study guides and review books, as well as to attend review sessions or seek help from a tutor.

Q4: How are the multiple-choice questions on the AP Chemistry exam scored?

The multiple-choice questions on the AP Chemistry exam are scored based on the number of correct answers. Each question is worth one point, and incorrect answers do not result in a deduction of points. This means that it is important to answer all questions, even if you are unsure of the answer.

Q5: What is the best way to approach the AP Chemistry exam?

The best way to approach the AP Chemistry exam is to read each question carefully and make sure you understand what is being asked. It is also helpful to eliminate any obviously incorrect answer choices and then use your knowledge and critical thinking skills to choose the best answer from the remaining options. It is important to manage your time effectively and not spend too much time on any one question.

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