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kotreny
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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
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)
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.
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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