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Topic: Problem of the week - 12/11/2012  (Read 27006 times)

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Offline Borek

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Problem of the week - 12/11/2012
« on: November 12, 2012, 10:12:08 AM »
The boiling point of a substance A is 7°C. A is unstable, and if left turns into a compound B, which sublimates at 135°C. When heated B decomposes into a solid C (melting point 160°C), a liquid D (boiling point 67°C) and a gas E. The liquid D is not stable, and if left turns into a solid F (melting point 175°C).

Using table of molar and mass fractions determine formulas of all compounds, write all reaction equations, tell which compounds are ionic and which are covalent.
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Offline Dan

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Re: Problem of the week - 12/11/2012
« Reply #1 on: November 12, 2012, 05:55:27 PM »
That's a cool question. A nice demonstration of the chemistry of these compounds - I am somewhat amazed that I could recall it from the recesses of my brain. Hasn't been all that long though.
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Offline CrazyAssasin

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Re: Problem of the week - 12/11/2012
« Reply #2 on: November 13, 2012, 01:23:25 PM »
A,B - PCl2F3
C - PCl5
D - PCl4F
E - PF5
F - PCl4F
While speaking about bonding type all of them are covalent.

Offline Dan

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Re: Problem of the week - 12/11/2012
« Reply #3 on: November 14, 2012, 02:52:30 AM »
While speaking about bonding type all of them are covalent.

What's the difference between A/B and D/F then?
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Offline Borek

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Re: Problem of the week - 12/11/2012
« Reply #4 on: November 15, 2012, 04:27:35 PM »
Determining the formulas wasn't hard, but apparently further details are harder... Although some simple logic should help.
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Offline Big-Daddy

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Re: Problem of the week - 12/11/2012
« Reply #5 on: November 17, 2012, 05:08:32 PM »
The boiling point of a substance A is 7°C. A is unstable, and if left turns into a compound B, which sublimates at 135°C. When heated B decomposes into a solid C (melting point 160°C), a liquid D (boiling point 67°C) and a gas E. The liquid D is not stable, and if left turns into a solid F (melting point 175°C).

Using table of molar and mass fractions determine formulas of all compounds, write all reaction equations, tell which compounds are ionic and which are covalent.

The dashes indicate 0% content, correct? (Not "withheld knowledge")

Offline Borek

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Re: Problem of the week - 12/11/2012
« Reply #6 on: November 17, 2012, 05:09:50 PM »
Yes.
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Offline Borek

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Re: Problem of the week - 12/11/2012
« Reply #7 on: November 19, 2012, 11:50:48 AM »
Technically there is still no correct answer...
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Offline CrazyAssasin

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Re: Problem of the week - 12/11/2012
« Reply #8 on: November 19, 2012, 01:04:39 PM »
So, I think that B and F should be ionic compounds, because their intermolecular forces increases, while all the others are covalent.

Offline Borek

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Re: Problem of the week - 12/11/2012
« Reply #9 on: November 19, 2012, 02:32:57 PM »
Care to list ions?

These are really interesting compounds, quite surprising.
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Offline CrazyAssasin

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Re: Problem of the week - 12/11/2012
« Reply #10 on: November 19, 2012, 03:07:20 PM »
I give up, I can't understand how those compounds turn into ionic

Offline Dan

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Re: Problem of the week - 12/11/2012
« Reply #11 on: November 19, 2012, 06:10:48 PM »
Hint:

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Offline DrCMS

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Re: Problem of the week - 12/11/2012
« Reply #12 on: November 20, 2012, 04:15:15 AM »
Hint:



Very nicely hinted Dan - the correct answer without being obvious.

Offline Rutherford

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Re: Problem of the week - 12/11/2012
« Reply #13 on: November 26, 2012, 12:42:10 PM »
I hope that it is okay that I ask something again:
When determining the molar masses, I used compound C. I used the molar shares to get the formula XY5. Using mass shares, I got two equations, which are identical (x is the molar mass of X and y is of Y):
x/(x+5y)=0.1487
5y/(x+5y)=0.8513
Solving one of them: x=0.873y. When I put 35.5g/mol for y I get 31g/mol for x, but this is done on assumption. Can I calculate x and y without assumpting the molar masses?

I know that the question isn't solved, yet, but I am not asking that part. The part I am asking here is solved by CrazyAssasin.

Offline Borek

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Re: Problem of the week - 12/11/2012
« Reply #14 on: November 26, 2012, 12:49:27 PM »
Solving one of them: x=0.873y. When I put 35.5g/mol for y I get 31g/mol for x, but this is done on assumption. Can I calculate x and y without assumpting the molar masses?

No, you have to do it by trial and error.
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