[trinhn812]

Consider two different compounds, each with the formula C2H6O.  One of these compounds is a liquid at room conditions and the other is a gas.  Write Lewis structures.

So the only idea I have is that oxygen would have 2 bonds and 2 lone pair. I don't know how the structure would change?

[agrobert]

I think you are misinterpreting your own question.  There are two chemicals, one is a liquid the other is a gas.  The Lewis structures for these compounds are different because they are different compounds.

Look for a C₂H₆O as a

Liquid (alcohol)

Gas (ether)

You are right, oxygen has two bonds in both structures and two lone pairs but there is a difference.

Try the question now.

[Alpha-Omega]

OK C2H6O has a number of permutations.  I have attached the structures now you have to figure out the lewis dot structures.

The molecular formula isn't unique. You really would like a compound to have one and only one name, and for a name to refer to only one molecule. After thinking about it some more you realize that the molecular formula itself could be written several ways, like H6C2O (lightest element first) or OC2H6 (heavest element first). There are six possible permutations for three atoms.
Searching for the first alternative you come across lecture slide which says "H6C2O could correspond to both Ethanol (H3CH2COH) and dimethyl ether (H3COCH3)". Ahh! A clue! Maybe this is called ethanol. But it's kinda worrying to see the formula written as H3CH2COH, which is different than the six permutations listed above.
Further searching finds links to sites promoting the commercial use of ethanol, but not until the sixth link do you find some useful chemical information and verification that you've got the right structure. But it is still disconcerting that they use the formula CH3CH2OH which is yet another possibility.
What are you going to do the next time you want to find information about a molecule? It seems these things have names, so you look into that some more and find out that the International Union of Pure and Applied Chemistry (IUPAC to its friends and enemies alike) have a huge amount of documentation related to nomenclature. Using their rules gives a way to assign a unique name to a molecule.
And look, that page says ethanol is written C2H5OH. *sigh*.
The documentation is overwhelming so in growing frustation you find an introduction to the naming of compounds, which conviently uses ethanol as its example.
At its simplest, the IUPAC name for an organic compound contains these two parts:
•   a root indicating how many carbon atoms are in the longest continuous chain of carbon atoms.
•   a prefix and/or suffix to indicate the family to which the compound belongs.
The longest carbon chain is two carbons so it has the prefix "eth". There is a single bond between them (that's "single bond" as in a bond with bond order of 1, not that there's only one bond between them) so it's an "ethane". There's an OH on the end which uses the suffix "ol". Drop the "e" and join them to make "ethanol". Ta-da!
Upon reading that tutorial you realize there's a lot of memorization of names, and you went into physics because you prefered formulas and math over names. And because you would rather be electrocuted or irradiated instead of being around chemical containers with big warning stickers like "Danger: Bone Seeker" or "The toxicity of this substance has not yet been determined."
After digging around a bit you realize that even trained chemists have problems with names. Chemistry librarians were worth their weight in platinum in their knowledge of the arcane magic of finding the right literature references.
Good thing you've got a computer. There is software to help generate an IUPAC name. But my, the results sure looks complicated, the process is opaque (to non-experts and even non-specialists in a domain) and there's the fine print that "from time-to-time" some compounds can't be named because "some classes of compounds may not yet have systematic nomenclature definitions available."
Ethanol, also known as ethyl alcohol, drinking alcohol or grain alcohol, is a flammable, colorless, mildly toxic chemical compound, and is best known as the alcohol found in alcoholic beverages. In common usage, it is often referred to simply as alcohol. Its molecular formula is variously represented as EtOH, CH3CH2OH, C2H5OH or as its empirical formula C2H6O (which it shares with dimethyl ether).
After the use of fire, fermentation of sugar into ethanol is perhaps the earliest organic reaction known to humanity, and the intoxicating effects of ethanol consumption have been known since ancient times. In modern times ethanol intended for industrial use has also been produced from byproducts of petroleum refining.
Dimethyl ether, also known as methoxymethane, oxybismethane, methyl ether, wood ether, and DME, is a colorless gaseous ether with an ethereal odor. Dimethyl ether gas is water soluble. It has the formula CH3OCH3 or as its empirical formula C2H6O (which it shares with ethanol). Dimethyl ether is used as an aerosol spray propellant, and is used in conjunction with propane to give a thermic expansion that lowers temperature to -60°C. This property is usefull in cryogenic freezing of common warts found on the human body. Dimethyl ether is also a clean-burning alternative to liquified petroleum gas, liquified natural gas, diesel and gasoline. It can be made from natural gas, coal, or biomass.

[agrobert]

This is more confusing than helpful and you gave trinhn812 the answer.