Chandler H.

asked • 11/11/14

How do you find the number of C and O atoms in a hydrocarbon when it says it weighs 88 g/mol?

How do you find the number of C and O atoms in a hydrocarbon when it says it weighs 88 g/mol?

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Dal J. answered • 11/11/14

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Dal J.

Okay, I can draw a molecular diagram of C7H4 - it has 2 Hydrogen-Carbon bonds at each end, for a total of 4 Carbon bonds, then double-bonds between each pair of Carbon, for a total of 24 Carbon bonds.  That's legal and physically possible, being a fully-polyunsaturated heptene -- hept-123456-ene, for gosh sake -- but it's not a typical chemical at this level of chemistry. 
 
I wouldn't say it didn't exist, but I would say that it certainly won't be found in a Chem 1 class!
 
With no alteration in the question, that's your answer.
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11/12/14

Harvey F.

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Based on the description of the molecule, it seems there are only 6 double bonds between the pairs of carbon atoms with 4 single bonds (two on each end) for a total of 16 carbon bonds. Otherwise this is a very interesting answer!
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11/14/14

Dal J.

Sorry, imprecise wording.  The 24 number was the total number of sides in the carbon-carbon bonds. Your number of 16 is more accurate, since it's the number of electrons that are virtually counted as whipping around multiple atoms. 
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11/15/14

Evan H. answered • 11/11/14

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Dal J.

Certainly, that H4C7 answer I found doesn't feel like a reasonable hydrocarbon.  You'd have 24 carbon-carbon bonds and only 4 carbon-hydrogen bonds.
 
But, 2 grams per mole difference in molecular weight from the problem statement isn't reasonable, either. 
 
My guess is that the word "hydrocarbon" was assumed into the question by the writer, and it's some other kind of organic molecule...although that means that more information would be required for a reasonable answer, too. 
 
Sure was a fun mind-stretch, though.
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11/11/14

Evan H.

Not saying it is, just saying that assuming a hydrocarbon nothing in existence weights 88g/mol. There are one of two things wrong with the problem statement: the hydrocarbon definition or the molar mass. Considering there are two unknowns(unless you just guess at Oxygens) and it's against the definition of a hydrocarbon. I vote for the molar mass being wrong.
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11/11/14

Dal J.

Well, I can draw a molecular diagram of C7H4 - it has 2 Hydrogen-Carbon bonds at each end, for a total of 4 Carbon bonds, then double-bonds between each pair of Carbon, for a total of 24 Carbon bonds.  That's legal and physically possible, being a fully-polyunsaturated heptene -- hept-123456-ene, for gosh sake -- but it's not a typical chemical at this level of chemistry.  I wouldn't say it didn't exist, but I would say that it certainly won't be found in a Chem 1 class!
 
So, with no alteration in the question, that would be the "correct" answer.  I think you're likely correct that the question SHOULD have had a molecular weight of 86, or the word "hydrocarbon" should have been "carbohydrate".
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11/12/14

Dal J.

Oh, among straight-chains, hept-135-yne qualifies as well. 
 
I'm sure there's a methyl benzyne molecule that would qualify, although I won't attempt to name the darn thing.  Methyl attachment at 1, double bond at 1 and triple bonds at 3 and 5. 
 
Like you said,  CnH(2n+2-2d), where d is the degree of unsaturation(double bonds, rings, etc).
 
To answer the question as asked, if n is 7, then 2d has to be 12 and there have to be 6 extra "unsaturated" bonds somewhere in the chain or ring or whatever.
 
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11/12/14

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