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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

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.

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.

A,B  PCl2F3
C  PCl5
D  PCl4F
E  PF5
F  PCl4F
While speaking about bonding type all of them are covalent.

While speaking about bonding type all of them are covalent.
What's the difference between A/B and D/F then?

Determining the formulas wasn't hard, but apparently further details are harder... Although some simple logic should help.

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")

Yes.

Technically there is still no correct answer...

So, I think that B and F should be ionic compounds, because their intermolecular forces increases, while all the others are covalent.

Care to list ions?
These are really interesting compounds, quite surprising.

I give up, I can't understand how those compounds turn into ionic

Hint:
(https://www.chemicalforums.com/proxy.php?request=http%3A%2F%2Fupload.wikimedia.org%2Fwikipedia%2Fcommons%2Ff%2Ffc%2FTetrahedron.svg&hash=9a330de528c541d710b00ace8ba03558)(https://www.chemicalforums.com/proxy.php?request=http%3A%2F%2Fupload.wikimedia.org%2Fwikipedia%2Fcommons%2F0%2F07%2FOctahedron.svg&hash=a8d9fbda26ea2e90628539d62a54fda5)

Hint:
(https://www.chemicalforums.com/proxy.php?request=http%3A%2F%2Fupload.wikimedia.org%2Fwikipedia%2Fcommons%2Ff%2Ffc%2FTetrahedron.svg&hash=9a330de528c541d710b00ace8ba03558)(https://www.chemicalforums.com/proxy.php?request=http%3A%2F%2Fupload.wikimedia.org%2Fwikipedia%2Fcommons%2F0%2F07%2FOctahedron.svg&hash=a8d9fbda26ea2e90628539d62a54fda5)
Very nicely hinted Dan  the correct answer without being obvious.

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 XY_{5}. 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.

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.

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.

The trigonal bypyramidal compounds will always have 2 axial and 3 equatorial substituents, so you can't get isomers for XY_{5}.
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.

I didn't mean for XY_{5}. I meant it for compounds A/B and D/F. There is more that one possible space arrangement for XY_{4}Z.
I can't see how the tetrahedron and the octahedron are related to the geometry of these molecules, so I will leave this.

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.

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.

I came to the idea that the molecules combine
You're so close now. How could XY_{5} (represented as a trigonal bipyramid) combine to form a tetrahedron and an octahedron?
Hint: How many vertices do these polyhedra have?

Trigonal bipyramid has 5, tetrahedron 4 and octahedron 6. From two trigonal bipyramids, one tetrahedron and one octahedron can be formed. If XY_{5} would have to make a tetrahedron, then it will lose one Y so it will become a cation (XY_{4}^{+}). If XY_{5} would have to make a octahedron it has to accept a Y so it will become an anion (XY_{6}^{}). These two can combine to make an ionic compound XY_{4}*XY_{6} or X_{2}Y_{10}.

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?

Do they overlap?

Do they overlap?
I don't understand your question.

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.

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 PCl_{5} and E is PF_{5}
A and D are covalent and could autoionise 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 phosphorusfluorine bond would be stronger (PF_{5} doesn't autoionise but PCl_{5} does)
So B would be [PClF_{3}]^{+}[PCl_{3}F_{3}]^{} and F would be [PCl_{3}F]^{+}[PCl_{5}F]^{} right?