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

Question

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

Answer 1

Chandler,

The hydrocarbon, Hexane C₆H₁₄ , has a molar mass of 86g/mol  which is very close to the 88g/mol you specified. Your question asks how many atoms of C and O? Hydrocarbons by definition have just Hydrogen and Carbon, not Oxygen. Are you sure the question you asked is accurate and complete?

Answer 2

Okay, 88 g/mol tells you that the total molecular weight of the atoms in the molecule will add up to 88.

 

Now, the molecular weight of carbon is just over 12, and the molecular weight of oxygen is just under 16.  The number of atoms of each in a molecule must be a positive integer. 

 

12c + 16o = 88  where c, o are in  [1,2,3,4,5...]

 

3c + 4o = 22  so c must be even (or the left side of the equation is odd)

                     and c must be less than 7 [2,4,6]

 

o = (22 - 3c) / 4

 

 

If C is 2, then O is (22-6)/4 = (16/4) is 4

If C is 4, then O is (22-12)/4 = (10/4) is not an integer

If C is 6, then O is (22-18)/4 = (4/4) is 1

 

There are two possible formulae

 

C2O4

 

C6O

 

 

Of course, I get down here and ask myself where are the hydrogen?  The Hydro in hydrocarbon  is for hydrogen, and there MIGHT be oxygen in a hydrocarbon molecule -  but there aren't always.  No, in fact a ketone wouldn't be a pure hydrocarbon, it's a different kind of organic molecule.   

 

Are you sure you read the problem right?  Are there any other words involved?

 

 

Okay, from scratch. 

 

The molecular weight of carbon is 12, so there are, at most, 7 carbons in the chain, weighing 7*12 = 84.  That leaves a molecular weight of 4 for the hydrogen, giving us H4C7 as one possible formula.  That's possible, but it's a weird formula - too few hydrogen for my taste.

 

If there are 6 carbons in the chain, we have 88 - 6*12 = 16 hydrogen, giving H16C6.  

 

Now, normally in a simple hydrocarbon, there are 3 hydrogen on each end carbon and 2 in each center carbon, for a maximum number of 14 hydrogen for 6 carbon.  I really dislike more hydrogen than that, because I don't remember that being possible for normal carbon valences.

 

I'm stumped how either of these answers makes sense.   

 

Okay, C7H4 is legal.  See comments.   

 

 

 

 

 

 

 

Answer 3

The definition of a hydrocarbon is that is consists of only Carbon and Hydrogen atoms, this tells us that O = 0

 

Now once we found that, the general equation for an unsaturated hydrocarbon is CnH2n+2.

 

So,

 

12 * n + 1(2n+2) = 88

 

14 * n + 2 = 88

14 * n = 86

n = 86/14 = 6.1 = 6 Carbon atoms/molecule

 

This adds up to a little over 86g/mol, which is not exact. But it's the only hydrocarbon in the range.

If they misunderstood what a hydrocarbon was, then the answer is most likely pentanol or C5H12O which has a molecular weight of 88. You can find this with the same method as above, just keep varying the amount of O.

 

P.S. A non ionic hydrocarbon can only have the formula CnHn+2-2d, where d is the degree of unsaturation(double bonds, rings, etc). Anything more than that will break the octet on a carbon(duet on hydrogen) and anything less leaves charges. Therefore, any odd number of hydrogens involve carbanions or carbocations which are not really all that valid as they are extremely reactive.