USNCO Exam Annotations: Comments and corrections

[LilyRaines]

Here you can share with our editor and volunteers any comments, suggestions, or corrections you have after reviewing the annotated solutions shared on the USNCO Exams Page.

Please start your comment with the exam and year you are referencing - i.e. Local 2020, National 2019.

[AndrewYe]

For question 50 on the 2017 national Part I exam, the exam annotations indicate that for part (B) the molecule whose Lewis structure is shown is C2H4, when the problem states that the comparison is between C2H6 and B2H6. The annotations give the correct answer, it's just that the different Lewis structure compared to the test answer might confuse some people. 

[Dr_Chen]

Well received, thank you for pointing it out!

Dr. Chen

[Dr_Chen]

A few typos were found in national 2020 annotation by a reader as follows, thanks!

I noticed a couple of minor mistakes in the annotated solutions for National 2020.

Q24: It could just be that the author didn't balance the equation, but the dissociation equation for Ag2SO4 is missing a coefficient of 2 in front of Ag+. 

Q28: The solution says that ka = 0.00411, but 0.0114/2.79 = 0.00409.

Q29: This is very minor, but this equation is missing an open bracket around the second A.

Q30: Just a wording thing. The word should be "consistent"/"inconsistent" not "constant"/"inconstant" with regards to which of I and II are inconsistent with the mechanism. 

[Dr_Chen]

typo in 2019 national from reader, thank you!

Hi Dr. Chen, I see a typo in 2019 National #6. The volume of acid should be 45 + 50 + 55 = 150 so the molarity should be .0967 M. The solutions had 45 + 50 + 55 = 145, which isn't a big deal but it is a typo

[Dr_Chen]

One more tiny bug found for Q18 of national 2019, the Ksp=4S^3 for Li2CO3 rather than S^2. The revised annotation is attached, and be shown in the next update.

[AaronMSimmons]

National Exam 2020 Part 2: Question #2 part c, ii

I am not understanding why the answer key explains that an extra 0.0100 mol of NaOH must be added after already utilizing the Henderson Hasselbach in the expression involving the 3rd dissociation constant.  I follow everything up to the point where the key states "BUT WAIT!"  When I worked this out on my own, I stopped before that section of the key, so my answer ended up being 0.0283 mol NaOH.  My students made note that my solution did not agree with the ACS key, but I could not explain to them why that extra 0.0100 mol of OH- need to be added.  Can anyone explain to me why this is the case?  Any help would be greatly appreciated.  Thank you very much for your time.