I encountered this problem during an assignment:

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I answered it as C, but the assignment grader marked it as wrong and indicated that A is correct.

I asked another instructor about it and gave me a different answer: D.

I am confused on how to approach this problem given that I have received different answers from what I consider reputable sources. How do I proceed to solve this?

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    $\begingroup$ Hint: Lighter gases diffuse faster than heavier gases! $\endgroup$ – Ed V Apr 8 at 23:15
  • $\begingroup$ I suggest you to better change the institution, if possible ;) $\endgroup$ – Zenix Apr 8 at 23:57
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    $\begingroup$ @Zenix Having administered multiple choice exams to many general chemistry students over the years, the occasional bad question happens. Example of an inadvertantly bad question: Which compound (of the listed ones on the exam) has the most polar bonds? Some students will look for the listed compound that had bonds that are most polar. That was the intent of the question. But others will count the number of polar bonds per compound and rank the listed compounds that way. Not unreasonable, actually. $\endgroup$ – Ed V Apr 9 at 1:54

No answer is correct, but "A" is closest to the correct answer. Heavier molecules diffuse slower than lighter ones because the same amount of thermal energy has to move more mass. However, this relation (diffusion rate is inversely proportional to the square root of molecular weight) actually leads to 44 seconds not 64. The professor who told you "D" probably misunderstood that in the second chamber there is HCl (acidic) instead of ammonia (basic).

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  • $\begingroup$ I am getting 44s, still not 60s.. $\endgroup$ – Zenix Apr 8 at 23:21
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    $\begingroup$ It should definitely be slower. Actualy I got just 60 s at first ... because I mistook the molecular weight of HCl for that of Cl2. Maybe they also did so. Good catch, will edit. $\endgroup$ – Oscar Lanzi Apr 8 at 23:24
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    $\begingroup$ @Zenix Yeah, they forgot the square root in Graham’s law. But it was really a logic problem! $\endgroup$ – Ed V Apr 8 at 23:24

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