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Topic: Need help on a Chem Olympiad problem  (Read 5444 times)

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Offline Sophia7X

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Need help on a Chem Olympiad problem
« on: March 02, 2012, 11:30:38 PM »
http://www.chemteam.info/NChO/NChO-99-National.html

Number 3, the answer is supposed to be B.
I thought it was A since delta T is final - initial. And final = highest point on graph, and initial = lowest point?
 34-24 = 10, but the answer is supposed to be 12.
Entropy happens.

Offline Vidya

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Re: Need help on a Chem Olympiad problem
« Reply #1 on: March 03, 2012, 02:38:25 AM »
here the graph is extrapolated to compensate the heat lost by the water in exothermic reactions to the surroundings as the reaction proceeds.This shows the rise in temperature that would have taken place had the reaction been instantaneous.
on extrapolation of the graph in the question- t(final) - 36 deg C   

Offline Borek

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Re: Need help on a Chem Olympiad problem
« Reply #2 on: March 03, 2012, 04:38:01 AM »
Nice example Uma.

I don't like the question. Plot is rather lousy and selecting 12 is questionable to me. Why not 14? I could probably even manage to draw a reasonable line that will give answer 15 (C).

Edit: this is not the only question I don't like. Question 4 - I have no idea which method will be the best, I would try both spectroscopy and conductometry, to check which gives better results. Question 6 - answer is almost three times larger than the Ba(OH)2 solubility at the room temperature. No idea if it gets better later.
« Last Edit: March 03, 2012, 04:52:20 AM by Borek »
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Offline Sophia7X

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Re: Need help on a Chem Olympiad problem
« Reply #3 on: March 03, 2012, 07:49:04 AM »
Thank you.

It actually gets slightly better in the later years, this is 1999. There are some obscure questions in pretty much every Chem Olympiad test ("how do you expect a high school student to know that" type of questions).

4.) I got spectroscopy simply because I remember doing a lab with FeSCN+2 and a Spec 20. I don't know what conductometry is though.
Entropy happens.

Offline Borek

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Re: Need help on a Chem Olympiad problem
« Reply #4 on: March 03, 2012, 08:41:10 AM »
4.) I got spectroscopy simply because I remember doing a lab with FeSCN+2 and a Spec 20. I don't know what conductometry is though.

I am afraid that's what happened - whoever asked teh question thought just in terms "we do it this way so students should know it is done this way" - but just becaiuse it is done this way doesn't mean it is the only (and the best) approach.

Conductometry is a technique based on measurement of the solution conductivity (reciprocal of resistance). Each ion present in the solution lowers the resistance. Each ion has its own specific molar conductivity, and for diluted solutions conductivity of solution is just a sum of conductivities of all ions present - these in turn depend on the the concentration. While it is not a good method of checking concentration of each ion separately, varying concentrations of substances present and measuring changes of the conductivity of the solution can give a lot of information about how the ions interact, as their exact concentrations depend on the equilibria.

If you are preparing for Chemistry Olimpiad, check our Problem of the week - I am posting a new question each week on Monday in the Undergraduate forum. See the previous ones:

http://www.chemicalforums.com/index.php?topic=56587.0
http://www.chemicalforums.com/index.php?topic=56323.0
http://www.chemicalforums.com/index.php?topic=56059.0
http://www.chemicalforums.com/index.php?topic=55810.0
http://www.chemicalforums.com/index.php?topic=55625.0
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