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Topic: Reason for error  (Read 13789 times)

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Offline Sophia7X

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Reason for error
« on: April 25, 2012, 02:15:48 PM »
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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Offline JGK

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Re: Reason for error
« Reply #1 on: April 25, 2012, 03:18:46 PM »
There is another far more obvious explanation

Read the Wiki and see if you can spot it -http://en.wikipedia.org/wiki/Magnesium_oxide
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Offline Sophia7X

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Re: Reason for error
« Reply #2 on: April 25, 2012, 04:12:57 PM »
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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Offline Arkcon

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Re: Reason for error
« Reply #3 on: April 25, 2012, 05:05:21 PM »
Was something done to remove magnesium nitride?  As I recall, there was a pretty simple step to remove it.
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Offline Sophia7X

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Re: Reason for error
« Reply #4 on: April 25, 2012, 05:12:32 PM »
Nope
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Offline Olympiad_Tutor

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Re: Reason for error
« Reply #5 on: April 26, 2012, 02:37:42 AM »
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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Offline JGK

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Re: Reason for error
« Reply #6 on: April 27, 2012, 04:55:26 PM »
Quote from: JGK on April 25, 2012, 03:18:46 PM
There is another far more obvious explanation

Read the Wiki and see if you can spot it -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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Offline Olympiad_Tutor

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Re: Reason for error
« Reply #7 on: April 28, 2012, 07:29:59 AM »
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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Offline ramboacid

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Re: Reason for error
« Reply #8 on: April 29, 2012, 04:52:24 PM »
Quote from: Ch_Olympiad_Winner on April 28, 2012, 07:29:59 AM
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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Offline cheese (MSW)

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Re: Reason for error
« Reply #9 on: May 01, 2012, 05:29:04 PM »
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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Offline Olympiad_Tutor

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Re: Reason for error
« Reply #10 on: May 03, 2012, 02:03:36 AM »
Quote from: cheese (MSW) on May 01, 2012, 05:29:04 PM
...
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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Offline cheese (MSW)

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Re: Reason for error
« Reply #11 on: May 03, 2012, 12:29:37 PM »
Ch-Olympian-champion: I was teasing you.  I see you have added BS and PhD Biochem
to your credentials: impressive!
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Offline Olympiad_Tutor

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Re: Reason for error
« Reply #12 on: May 04, 2012, 03:14:54 AM »
Quote from: cheese (MSW) on May 03, 2012, 12:29:37 PM
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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Offline Sophia7X

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Re: Reason for error
« Reply #13 on: May 04, 2012, 06:22:49 PM »
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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Offline cheese (MSW)

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Re: Reason for error
« Reply #14 on: May 04, 2012, 08:13:57 PM »
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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