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MCAT Chem Question — Empirical Formula

One mole of a compound consists of approximately 72g carbon and 6g hydrogen. What is its empirical formula?

A. C6H6


C.  CH

D. C6H12

Click for Explanation

The empirical formula expresses the smallest number of atoms making up a molecule in the correct integer ratio. In this question, converting to moles gives us six moles of carbon and six moles of hydrogen in one mole of the entire compound. This gives us our molecular formula, C6H6. However, remember that empirical formulas must consist of the smallest ratio of all atoms involved! This ratio is CH, which is our answer.

A. C6H6, incorrect. This represents the molecular, rather than the empirical, formula.

B. CHO, incorrect. There is no oxygen present.

C.  CH, correct. The empirical formula is the smallest whole-number ratio of the elements making up a molecule.

D. C6H12, incorrect. This does not reflect the correct molar ratio of the elements.

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