# Chemical Forums

## Chemistry Forums for Students => Undergraduate General Chemistry Forum => Problem of the Week Archive => Topic started by: Borek on November 12, 2012, 10:12:08 AM

Title: Problem of the week - 12/11/2012
Post by: Borek 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.
Title: Re: Problem of the week - 12/11/2012
Post by: Dan 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.
Title: Re: Problem of the week - 12/11/2012
Post by: CrazyAssasin 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.
Title: Re: Problem of the week - 12/11/2012
Post by: Dan 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?
Title: Re: Problem of the week - 12/11/2012
Post by: Borek 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.
Title: Re: Problem of the week - 12/11/2012
Post by: Big-Daddy 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")
Title: Re: Problem of the week - 12/11/2012
Post by: Borek on November 17, 2012, 05:09:50 PM
Yes.
Title: Re: Problem of the week - 12/11/2012
Post by: Borek on November 19, 2012, 11:50:48 AM
Technically there is still no correct answer...
Title: Re: Problem of the week - 12/11/2012
Post by: CrazyAssasin 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.
Title: Re: Problem of the week - 12/11/2012
Post by: Borek on November 19, 2012, 02:32:57 PM
Care to list ions?

These are really interesting compounds, quite surprising.
Title: Re: Problem of the week - 12/11/2012
Post by: CrazyAssasin on November 19, 2012, 03:07:20 PM
I give up, I can't understand how those compounds turn into ionic
Title: Re: Problem of the week - 12/11/2012
Post by: Dan on November 19, 2012, 06:10:48 PM
Hint:

Title: Re: Problem of the week - 12/11/2012
Post by: DrCMS on November 20, 2012, 04:15:15 AM
Hint:

Very nicely hinted Dan - the correct answer without being obvious.
Title: Re: Problem of the week - 12/11/2012
Post by: Rutherford 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.
Title: Re: Problem of the week - 12/11/2012
Post by: Borek 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.
Title: Re: Problem of the week - 12/11/2012
Post by: Rutherford on November 26, 2012, 01:48:42 PM
I got an idea here. It is obvious that A/B and D/F are isomers. As the compounds are trigonal bipyramidal it was hard to find the appropriate names of these isomers, they can't be cis/trans nor mer/fac. Then I found that stereoisomers of trigonal bipyramidal molecules can be axial and equatorial, so I believe that they much differ in stability and therefore in their chemical properties. The only problem is that I don't know which form is more stable and which is covalent and ionic.
Title: Re: Problem of the week - 12/11/2012
Post by: Dan on November 26, 2012, 02:15:00 PM
The trigonal bypyramidal compounds will always have 2 axial and 3 equatorial substituents, so you can't get isomers for XY5.

The trigonal bipyramidal compound could be represented as:
(https://www.chemicalforums.com/proxy.php?request=http%3A%2F%2Ft1.gstatic.com%2Fimages%3Fq%3Dtbn%3AANd9GcQsK41UmQuzTXhprZXr0mmVauJ0JpbI661D1sMz_XFOQhkFQOJ_xA&hash=ea49591391aba364399ceb205aa6fb8f)

Think about the polyhedra in the hint I posted.
Title: Re: Problem of the week - 12/11/2012
Post by: Rutherford on November 27, 2012, 09:08:10 AM
I didn't mean for XY5. I meant it for compounds A/B and D/F. There is more that one possible space arrangement for XY4Z.
I can't see how the tetrahedron and the octahedron are related to the geometry of these molecules, so I will leave this.
Title: Re: Problem of the week - 12/11/2012
Post by: Dan on November 27, 2012, 09:49:35 AM
I can't see how the tetrahedron and the octahedron are related to the geometry of these molecules, so I will leave this.

Hint: empirical vs molecular.
Title: Re: Problem of the week - 12/11/2012
Post by: Rutherford on December 08, 2012, 01:01:38 PM
As no one tried this and as I am really curios about the answer, I will try it again.
You said empirical vs molecular, and on the picture there were a tetrahedron and an octahedron. I came to the idea that the molecules combine, so that B and F make molecular crystal cells which is the reason why they are solids.
Title: Re: Problem of the week - 12/11/2012
Post by: Dan on December 10, 2012, 06:12:42 AM
Quote
I came to the idea that the molecules combine

You're so close now. How could XY5 (represented as a trigonal bipyramid) combine to form a tetrahedron and an octahedron?

Hint: How many vertices do these polyhedra have?
Title: Re: Problem of the week - 12/11/2012
Post by: Rutherford on December 10, 2012, 07:53:05 AM
Trigonal bipyramid has 5, tetrahedron 4 and octahedron 6. From two trigonal bipyramids, one tetrahedron and one octahedron can be formed. If  XY5 would have to make a tetrahedron, then it will lose one Y so it will become a cation (XY4+).  If XY5 would have to make a octahedron it has to accept a Y so it will become an anion (XY6-). These two can combine to make an ionic compound XY4*XY6 or X2Y10.
Title: Re: Problem of the week - 12/11/2012
Post by: Dan on December 10, 2012, 08:35:23 AM
Correct, phosphorous(V) halides can undergo autoionisation.

Se for example: http://en.wikipedia.org/wiki/Phosphorus_pentachloride#Structure

So, can you propose structures for B and F?
Title: Re: Problem of the week - 12/11/2012
Post by: Rutherford on December 10, 2012, 08:44:42 AM
Do they overlap?
Title: Re: Problem of the week - 12/11/2012
Post by: Dan on December 10, 2012, 12:21:30 PM
Do they overlap?

I don't understand your question.
Title: Re: Problem of the week - 12/11/2012
Post by: Rutherford on December 10, 2012, 12:33:15 PM
Because of the positive charge on one P atom and the negative on the other one, a ionic bond will be formed. As the ionic bond is shorter than the covalent bond, the tetrahedron and the octahedron should overlap, but then the electron pairs would repel each other much, so I am not sure about the structure.
Title: Re: Problem of the week - 12/11/2012
Post by: kaliaden on January 05, 2013, 03:48:21 AM
It's easy to find the empirical formula of each compound but how exactly did you calculate the molecular masses of X, Y and Z?

So far, i get that C is PCl5 and E is PF5

A and D are covalent and could auto-ionise to form B and F (which are ionic) respectively
I think it would be easier for a chlorine atom to move between two molecules of A and F since the phosphorus-fluorine bond would be stronger (PF5 doesn't auto-ionise but PCl5 does)
So B would be [PClF3]+[PCl3F3]- and F would be [PCl3F]+[PCl5F]- right?