Chemical Forums
Chemistry Forums for Students => High School Chemistry Forum => Topic started by: Sophia7X on April 25, 2012, 02:15:48 PM
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to determine the the formula of magnesium oxide, a student burns magnesium metal which forms a grey white solid. He determined it to be Mg5O4, what could be the cause of the error?
Ive narrowed it down to
1.)Magnesium reacted with the nitrogen in the atmosphere to form magnesium nitride
2.)Magnesium peroxide and oxide are formed
Leaning toward #1, but not sure... Could peroxides form from combustion anyway?
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There is another far more obvious explanation
Read the Wiki and see if you can spot it -http://en.wikipedia.org/wiki/Magnesium_oxide (http://en.wikipedia.org/wiki/Magnesium_oxide)
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MgO being hygroscopic could be a source of error but this is multiple choice so it's gonna be one of the provided choices :P
It's either the nitride is formed, or the peroxide... To get Mg5O4 instead of MgO as the formula you would have to lose oxygen, and the mass of oxygen is [mass of white solid - mass of magnesium] so the mass of the white solid has to be less than expected. I'm not sure whether MgO2 or Mg3N2 will do that.
isn't magnesium nitride yellow-green? But the question says grey white solid so I'm really confused because I've never heard of peroxides forming as a product of combustion
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Was something done to remove magnesium nitride? As I recall, there was a pretty simple step to remove it.
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Nope
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a simple approach to solve this without much calculation is to notice that we need to find a compound that has higher than 1:1 molar ratio of Mg:O or Mg:"O equivalent".
Now if we notice that the atomic weights of N and O are very similar (14 vs. 16) then we can think of Mg3N2 as ~"Mg3O2".
A combination of MgO and ~"Mg3O2" can give us the deduced ratio in Mg5O4, while adding Mg peroxide (MgO2) can only result in lower than 1:1 ratio for Mg:O.
Conclusion: Mg3N2 contamination
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There is another far more obvious explanation
Read the Wiki and see if you can spot it -http://en.wikipedia.org/wiki/Magnesium_oxide (http://en.wikipedia.org/wiki/Magnesium_oxide)
Everyone seems to be missing the following (from the WIKI):
Other: MgO is used as an insulator in industrial cables, as a basic refractory material for crucibles and as a principal fireproofing ingredient in construction materials.
Could it be that the formation of MgO on the outer surface of the magnesium actually prevents total magnesuim oxidation of the sample (due to its insulatory properties perhaps)?
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probably not much Mg survives given the extremely exothermic nature of the reaction.
Mg contamination may not even have been an option on this multi-choice question. Even it were, I would still go for the nitride since it is a well-documented reaction.
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probably not much Mg survives given the extremely exothermic nature of the reaction.
Mg contamination may not even have been an option on this multi-choice question. Even it were, I would still go for the nitride since it is a well-documented reaction.
This question was taken from this year's US Chemistry Olympiad multiple choice section, which I took a little over a week ago. Two of the four possible answer choices were posted by Sophia7X earler. I do remember that the last answer choice was something about overheating the MgO, if that makes a difference.
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If you’re are an “Olympian” of chem here is how your thought processes should have developed:
As an Olympian you should know that magnesium burns in air with dazzling brilliance to give MgO and Mg3N2. Hence its use in flash bulbs/flares.
This is multiple choice with only one correct answer;
1.)Magnesium reacted with the nitrogen in the atmosphere to form magnesium nitride
Stop right there! That is the correct answer so mark it as such and move to the next question (two min?). Gives you more time for the difficult questions.
Ignore other considerations they are not relevant. MgO has a high lattice energy (M^2+O^2-; small ions) and hence one of the highest mps known (>2800°C) for an ionic solid, hence its use as a refractory. Does not react with moisture or CO2 in air.
Now only an aficionado will know that MgO2 is difficult to prepare, but you should be able to use your chem intuition to realize that the process MO2 →Δ→ MgO + ½ O2 won’t be prevented at the Ts involved (note huge gain in lattice energy).
I believe Mg5O4 is put there as a subterfuge to distract all but the best students. Ignore it - is a red herring!
Now if they asked what is the fraction of Mg3N2 in the “Mg5O4” sample that is a different matter (answer should take you about 10 mins max).
Ch-Olympian-champion gets part marks. :)
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...
Ch-Olympian-champion gets part marks. :)
Olympiad Champ gets full marks (since he chose Mg3N2) and the extra credit for being careful. 8)
I do agree that the problem is somewhat of an overshoot with all the info given. Still, the creators of the problem succeeded in confusing a few students and even a couple of professors from what I heard. So the problem is a good one.
When I first saw the problem I did go for it from the reaction side but then I still checked the ratios.
One also needs to exclude the possibility of some Mg leftover ("reaction is too hot, violent, exothermic. etc.") if Mg contamination was an option.
The above steps can be applied during the few minutes available for the problem.
Those shooting for top 20 can only afford to lose a couple of points out of 60. This essentially means that every answer should be double checked and the rest of the multiple-choice answers eliminated.
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Ch-Olympian-champion: I was teasing you. I see you have added BS and PhD Biochem
to your credentials: impressive!
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Ch-Olympian-champion: I was teasing you. I see you have added BS and PhD Biochem
to your credentials: impressive!
Do you believe everything you read on the web? ::)
I prefer to think of myself as a former Olympian whose science knowledge is a bit rusty.
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They released the answer key to multiple choice and I got my test booklets back. I marked my answers on it so I think I got around a 50/60, given that I did not misbubble or something.
More than half of my errors were really stupid mistakes, like not converting units or calculation mistakes...
:( obviously did not make top 20, guess I can just hope for top 50 or 150. Better luck next time! And seriously, if I make the same type of mistakes next year I will slap myself XD
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You learn by your mistakes and it toughens you up!
You need a tutor. Many moons ago when I was in a similar situation
they brought in a grad student (senior undergrad?) to tutor me.
My school paid!!
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That's really nice! The two AP chem teachers at my school are always willing to help. I think I've spent at least 20 hours total of my teacher's time, asking endless questions after school. I didn't want to bother him TOO MUCH, so I tried to figure out many of the problems on my own... I'll say, a few of these problems took me hours to solve but I felt quite accomplished when I solved them on my own. :)
The types of problems I usually get wrong are the random facts (for example, this year asked what was a Grignard's reagant and what types of batteries are rechargeable; I had no idea). Guess the only way I can get these types of problems correct is to expose myself to even more chemistry. Also, stupid mistakes grrr... I just don't know where my brain was when I chose NH3 over CH3NH2 as a stronger base. Or when I forgot to convert atm to mmHg, and I even underlined the units the question wanted, haha.
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Sophia, you can certainly improve a lot in 1 year.
Analyze why you made those mistakes. Time pressure? Test anxiety? Work out a strategy to deal with those. Have you taken practice tests under timed conditions?
What my fellow Olympian Cheese said is certainly true. A tutor can help. I know examples where a tutor/teacher probably made significant difference for several students.
On the other hand, I never had a tutor and it worked out fine in the end. I definitely got many more bruises that way but it was a great learning experience.
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I don't know where I could get a tutor for this, but I think my teacher is sufficient. He's pretty dedicated, never refused me once when I asked him for help. Without him, I probably would have scored a 35 or something.
I need to learn my basic organic mechanisms as well, I think this year was the first year they included mechanisms in the free response, looks like they're starting to incorporating more advanced organic chemistry. Also will be working through Atkin's Chemical Principles over the summer.
And yes, I've taken them under timed conditions. Time was not a problem for me. It could be test anxiety but I didn't feel nervous, and I did pretty well on the local exam (with only one dumb mistake compared to 6 on this one).
Maybe my brain just wasn't present at that time... Next time I need to make sure I'm really really careful.
Could someone show me how to do #19, this was one of the problems I got wrong... I keep getting C??
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I am afraid AP teachers (whoever they are: I'm the product of a different system) may
not be up to the task. They are teachers and by definition may have an MSc at best. You need teachers
that are the brightest of the bright namely a PhD or soon to be a PhD. Professors of the future,
to train you to think outside the box. And whoever says that this can't be done is mistaken!
I should have added the grad student that tutored me was from probably the best school for
chemistry in the world at the time. And yes I passed that University's rigorous written and oral
exams and did my BSc there. (Hint: I had Wilkinson as one of my professors.)
So, go out and buy yourself Geoff Raynor-Canham's Descriptive Chemistry 5th ed and start reading
and learning. Grignard rgnts and batteries are all in there. (Dr R-C, "Geoff" was in the year
ahead of mine!) ;)
I am a great believer at struggling for hours on problems: helps the neurons to make shorter connections.
Not many students then or, especially, now do that.
If you had spent just two min of the Mg3N2 problem you would have had (perhaps!) time to check those units.
Always look at your answer to see that it make sense. If your calc density is 4.12×10^22 g cm^-3
you have probable forgotten Mr Avogadro's const (could be a neutron star?). Know Li has a density of 0.53 and Os 22.6
g cm^-3 so any density outside these values is wrong. etc etc
Practice, practice, practice -until it becomes second nature and from which comes confidence,
and nerves of steel. And above all, have fun!!
best wishes.
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Do you have a link to these Chem Olympics Exams?
Before embarrassing myself by making a claim on how well I would do on such an exam
I had better see one first! :).
(I would hope 58-60/60 in 8/10th of the time?)
How would your teacher score on the exam?
Tell me what Qu 19 is and I will answer it.
As you can gather I am not an Olympian. Olympians were the rookies in my system.
I took them to their BSc and PhDs. This is why I have a licence to tease Ch_Olympiad_Winner ;D
And for those of you that don't think they need a coach go watch Chariots of Fire about
probably the last guy (Scottish Christian) who won a Au medal (24 Olympics) without a
professional coach.
We are in it for the Au aren't we? I suspect you may have a special gift Sophia (you
have done a lot by yourself) so try and convince your parents to pay for a tutor.
Grad students are use to working for a pittance!
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US Chem Olympiad Exam Link (local and national): http://portal.acs.org/portal/acs/corg/content?_nfpb=true&_pageLabel=PP_SUPERARTICLE&node_id=1508&use_sec=false&sec_url_var=region1&__uuid=87591a12-f6d7-4e2c-8206-7a15418dfc7a (http://portal.acs.org/portal/acs/corg/content?_nfpb=true&_pageLabel=PP_SUPERARTICLE&node_id=1508&use_sec=false&sec_url_var=region1&__uuid=87591a12-f6d7-4e2c-8206-7a15418dfc7a)
there are some extremely affordable tutors with PhD by the way 8)
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Oh oops, forgot to provide a link!
#19 on the 2012 National test please.
http://portal.acs.org/portal/PublicWebSite/education/students/highschool/olympiad/pastexams/CNBP_029887
I think I recognize my error, that I cannot use the Cp of ice with an initial of temperature of > 0° C but I can't figure out how to set up the problem.
@cheese
Well, on the past exams I did for practice I scored from 45 to 58, 53 being the average. So I believe my teacher can do better, he's a smart guy.
Normally on these tests, I miss one calculation problem and 3-5 "trivia" questions. The rest of my errors are generally because I made a dumb mistake or I fell into a trick.
Anyway, I'll probably bug my teacher about doing the labs next year. I thought I had good enough lab experience so I didn't prepare for the lab portion. I spent a little too much time on one of the lab problems, and when I started the second one I only had 30 minutes left. Did not complete one of the 2 labs... Darn, I'm seriously the slowest titrator ever XD