Chemistry Quiz on empirical and molecular formula

[ghostanime2001]

ok I understand why now, because dry oxygen gas was used to burn the unknown sample in the analyzer

 

[epenguin]

Not so obvious. I agree the C/H proportions give you C₈H₂₀ which also fits the molar mass nicely. But as the OP said there are no C₈ compounds that can fit that ratio. There is too much H.

But even if you have oxygen in the compound, it doesn't help as far as I can see. If you replace any C-H with C-O-H you don't increase the H/C ratio. So i see just two ways to increase the H/C. One is by water of crystallisation. But then to be plausible you have to incorporate O into the molecule as well. Considering the molar mass I think this needs to be a C₄ molecule in which case you could fit the C/H ratio even without the water of crystallisation. There is an outside possibility with a C₆ molecule and water, but I can't see anything that adds up in either case.

I am taking it that any molar mass measurement that works at all will give you the MMass of compound without the water, and that we can take the accuracy of the measurements given as good.

 

[Yanick]

ok I understand why now, because dry oxygen gas was used to burn the unknown sample in the analyzer

That is good that you get this because at least you got something from this situation.

Not so obvious.

I am glad that I'm not the only one who is stumped. I haven't been able to figure it out myself.

Nothing that I've tried has seemed to work. Anything beyond C₆ doesn't fit the 2n+2 criteria. Trying C₄H₁₀ gets ugly numbers.

Maybe there is supposed to be some kind of other hetero-atom, but that seems like a silly possibility.

 

[ghostanime2001]

looks like we are all stumped on this question. Where do we go from here ?

 

[epenguin]

Maybe I've got it! Maybe we were meant to bring in something that was mentioned and then forgotten - nitrogen. When I include the possibility of N and use the well-known approximate whole number atomic masses there is a solution that I think is unique.

The only trouble is when I calculate it using accurate atomic masses I have disagreement with the given 116.26 by about 0.1 - a whole 1 in 1,000 discrepancy! Perhaps the OP could try the calculation with this idea, or someone else check the figures, or maybe I've overlooked some other solution?

That incredibly accurate molar mass figure given makes the problem look to me a contrived one. Surely a figure that accurate could not be result of experimental measurement; one would only calculate that figure once one knew the answer?

 

[ghostanime2001]

The question said carbon-hydrogen analyzer, and reading from the textbook "Chemistry 11" 2003 from Addison-Wesley it doesn't say anything about nitrogen concerning the bulbs that absorb carbon dioxide or water.

 

[epenguin]

The question said carbon-hydrogen analyzer, and reading from the textbook "Chemistry 11" 2003 from Addison-Wesley it doesn't say anything about nitrogen concerning the bulbs that absorb carbon dioxide or water.

All you have is the C/H ratio (you are not given the weight of original sample) and the molar mass of the compound. You have to try to home in on an answer just from those.

Experimental. Notoriously there are no good universal methods of classical chemical type for measuring total nitrogen. (You can fairly easily measure certain types of N such as primary amines.) I do not have the book you mentioned but it is probably something like here http://www.chm.davidson.edu/vce/stoichiometry/ch.html where it says "any nitrogen in the sample is reduced to N₂, which is unreactive" i.e. will leave no trace in measurements. I seem to vaguely remember doing an experiment like that at school; the CO₂ was measured gravimetically as an insoluble BaCO₃ precipitate I think.

Theoretical So you have a C/H ratio which you can take as accurate. You have the total molar mass. Although there is O in the CO₂ and H₂O you don't know how much of it was in the original molecules, so the measurements tell you nothing about this. And you don't know how much N there was.

But you also have atomic masses. And you have chemical principles. And from these you were already able to exclude one possiblity. And the fact that no one was able to find a solution when, I think, they considered only C, H and O is positive - it means the data are very constraining! So try and come up with something in as systematic a reasoning as possible (try and spell it out) involving C, H, O and N even if there remain some doubts like I have.
A hint is I think the fact the approximate whole number atomic masses of O and N are both even helps rapid elimination of some possibilities.

 

[ghostanime2001]

I have read that webpage that you posted. How would I determine the mass of the unknown element in the unknown compound ? Do I assume 100 g ? so then I can subtract the mass of carbon & hydrogen from 100 g to get the mass of the unknown element. But I don't know the molar mass of the compound :( this sucks...

 

[epenguin]

to be very honest, I am stumped on this one, from head to toe.

Most of the progress so far you made yourself so don't give up. Stumped by negative results is different from stumped by not trying. You got C₈H₂₀. But you realized that's is chemically impossible by itself. You cannot get that proportion of H to C with C and H alone. Not with C₂₀. But you can with a smaller molecule. Not just anything smaller, the ratios limit you to few possibilities. List them. But then a smaller molecule doesn't have mole mass around 116. But we are free to, and we have to, make up that mass with O and/or N atoms.

(I could also have got to the same conclusion saying adding O to my C₂₀ doesn't increase the possible H/C as explained previously. I can increase the ratio if I have N in the molecule. But that will make the MM greater than 116 too, unless the number of C is less.)

So try some possibilities and report even results that seem negative or we can't help you.

 

[ghostanime2001]

Assuming 100 g of the compound and including nitrogen in the sample, doesn't exactly make anything easier. By the way, what calculation made you believe that this is a chemically possible organic compound ?

 

[epenguin]

I have read that webpage that you posted. How would I determine the mass of the unknown element in the unknown compound ? Do I assume 100 g ? so then I can subtract the mass of carbon & hydrogen from 100 g to get the mass of the unknown element. But I don't know the molar mass of the compound :( this sucks...

Posts overlapped. You do have the molar mass of the compound, see #1.

 

[mfb]

@epenguin: While I can find a C,H,N,O combination with the approximate molar mass, how would that look like as a molecule? I think you get 1/2 of a bond somewhere, or a missing/additional hydrogen.

 

[ghostanime2001]

You're right

Posts overlapped. You do have the molar mass of the compound, see #1.

but how would I use that to determine the mass or molar mass of the unknown element ? and therefore the atomic mass of the unknown element ?

 

[epenguin]

@epenguin: While I can find a C,H,N,O combination with the approximate molar mass, how would that look like as a molecule? I think you get 1/2 of a bond somewhere, or a missing/additional hydrogen.

That's running ahead!

I'd think with the number of atoms we have to play with we will be able to think of many molecules within the rules.

 

[epenguin]

You're right
but how would I use that to determine the mass or molar mass of the unknown element ? and therefore the atomic mass of the unknown element ?

Just start with the fact that it can't be C₈H₂₀. But the ratio of number of atoms C/H is 8/20. So what are the possibilities for the number of C and H atoms in the molecule?

 

[mfb]

That's running ahead!

How could others follow (your advice) if you are not running ahead?

I'd think with the number of atoms we have to play with we will be able to think of many molecules within the rules.

I did not find a proper solution. Sure, you can make ions or radicals, but I doubt this is an intended solution.
I just have some doubts that this N addition will lead to something useful.

 

[ghostanime2001]

This was high school, radicals in quantitative problems was not in the curriculum. Let's not try to get off topic guys please...

 

[epenguin]

The OP seems to have abandoned, though the next step is very easy - we have established that there are less than 8 carbon atoms and I think most people know all the numbers that are less than 8.

That seems to me to swiftly lead to a unique answer that is anywhere near; it is as I said very near but not quite. Assuming this is the molecular formula I do after all find it difficult to think of a molecular one. Well I did find one that is notoriously semi-stable. I say one, but you are always going to get several for the price of one as you can move groups about. These searches could be a bit formalised, I am halfway doing that. Keep getting near, but there is a conspiracy giving me one H to many or too few - when you change anything you usually gain or lose two H, but I have not concluded.

 

[ghostanime2001]

searching for a compound with fewer than 8 carbons seems like painful work. There are thousands of organic compounds and several compounds could have similar molar mass. I don't think this question was meant to be more confusing than it's meant to be.

 

[mfb]

Keep getting near, but there is a conspiracy giving me one H to many or too few - when you change anything you usually gain or lose two H, but I have not concluded.

That is not just bad luck, it is a fundamental problem and the basis of my post #42.
You can calculate the number of hydrogen atoms needed if there are no rings and no double bindings - for the number of C, N O you assumed, this is an odd number. Every ring and every double binding reduces this number by 2 - you always keep an odd number, this does not fit to the even number you need to satisfy the C/H ratio together with the molecular mass.

That's why I asked if you can imagine a molecule with that. Otherwise, this direction leads nowhere.

searching for a compound with fewer than 8 carbons seems like painful work. There are thousands of organic compounds and several compounds could have similar molar mass. I don't think this question was meant to be more confusing than it's meant to be.

With just C N O H, a known C/H ratio and the total (small) molecular mass, it is not so hard to find all possible sum formulas.

 

[epenguin]

searching for a compound with fewer than 8 carbons seems like painful work. There are thousands of organic compounds and several compounds could have similar molar mass. I don't think this question was meant to be more confusing than it's meant to be.

Right now we are trying to establish a molecular formula, the numbers of atoms in the molecule, on the basis of information we have. This is really quite easy, certainly the next step. There are not thousands of numbers less than 7, there are only 7 of them! You know the ratio of H atoms to C atoms. On that basis what number of C atoms are possible?

Frankly after you get that you'll kick yourself. I think you'll do well then to ask yourself what was stopping you get it, it may be useful to you since you seem to be voluntarily going through old papers for revision or something. I would be interested to understand myself.

So try and answer the above question. If having tried you still can't say anything about that come back for more hints.

 

[ghostanime2001]

I know the reduced ratio of C is 2:5 from 8:20 ratio but I don't undestand what you said there about something having only 7 compounds with the above mentioned ratio.. to be quite honest, I don't really know if this question was meant to be this complicated. I guess I do need some more hints.

 

[epenguin]

I know the reduced ratio of C is 2:5 from 8:20 ratio but I don't undestand what you said there about something having only 7 compounds with the above mentioned ratio.. to be quite honest, I don't really know if this question was meant to be this complicated. I guess I do need some more hints.

I said essentially, several times now, the compound can have only 7, 6, 5, 4, 3, 2 or 1 C atoms.

Which of those are commpatible with your 2:5 C ratio?

 

[ghostanime2001]

2 carbon compound ?

 

[epenguin]

2 carbon compound ?

Yes, that appears compatible. Why? Are any others compatible? Which are incompatible?

 

[ghostanime2001]

Any others would not be compatible because the molecular formula is a integer multiple of empirical formula ?

 

[epenguin]

Any others would not be compatible because the molecular formula is a integer multiple of empirical formula ?

A true statement but I don't see any logic as an answer. But maybe we can do it this way.

What is the empirical formula of this compound? I mean just the C and H part of it.

 

[ghostanime2001]

empirical formula = C₂H₅

 

[epenguin]

empirical formula was C2H5

What then are possible molecular formulae in our case?

 

[ghostanime2001]

The possible molecular formulas may be:

1. C₄H₁₀ n=2
2. C₆H₁₅ n=3
3. C₈H₂₀ n=4