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Answer: (2)
Why? An empirical formula is a chemical formula that contains the lowest whole number of atoms for a class of compound. For example C2H4O2 can also be written CH3COOH, which shows the functional group as COOH or organic acid, the simplest of which is acetic acid. One possible method of working out the correct molecule is to create a diagram of the formula. Since carbon will share four of its electrons, it will only form four (4) single bonds. This is important when drawing an organic molecule to determine if the correct number of bonds is being formed.
Answering the Question:
To answer the question, it helps to determine if the whether the molecule does contain the lowest number of atoms.
Answer (1) C6H12O6 - is the general formula for most hexose sugars. A popular member of this group is the sugar glucose. Hexose sugars can be written:
CH=O-CHOH-CHOH-CHOH-CHOH-CH2OH
While this molecule does meet the requirement of being a molecular formula, it is not an empirical formula as it is not the lowest whole number of carbon atoms.
Answer (2) C2H4O2 - is both the empirical formula and molecular formula for ethanoic acid (acetic acid) and glycoaldehyde.
CH3-C=OOH - Ethanoic acid
OHCH2-CH=O - Glycoaldehyde
Both molecules contain the smallest whole number of atoms for their class,
Answer (3) C3H8O - This molecule is the molecular formula for the alcohol propanol. This molecule belongs to the group known as alcohols, with the functional group R-OH. However propanol is not the smallest of the alcohols. Methanol has the general formula CH3OH, which is also the empirical formula of alcohols.
CH3-CH2-CH2OH - Propanol
Answer (4) C4H8 - This molecule follows the molecular formula for alkenes, since it has four carbon atoms, then it would be most likely butene.
CH3-CH=CH-CH3 - 2-Butene
Butene C4H8 is a molecular formula, but again is not an empirical formula as member of the alkenes with the lowest count of atoms is ethene (C2H4). Therefore, answer (2) is the only answer that meets both the requirements of being a molecular formula and an empirical formula. If you did not know the possible classes of compounds a good guess would have been to choose the answer with the lowest number of carbon atoms and determine of all the carbon bonds were complete.
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