Find the empirical formula of the following compound: 0.77 mol of iron atoms combined with 1.0 mol of oxygen atoms.

The empirical formula is Fe₃O₄.

The empirical formula is the simplest whole-number ratio of atoms in a compound.

The ratio of atoms is the same as the ratio of moles, so our job is to calculate the molar ratio of Fe to O.

I like to summarize the calculations in a table.

Element   Moles    Ratio¹   ×3²   Integers³

    Fe         0.77       1         3             3

    O           1.0         1.3      3.9          4

¹ To get the molar ratio, you divide each number of moles by the smallest number (0.77).

² If the ratio is not close to an integer, multiply by a number (in this case, 3) to get numbers that are close to integers.

³ Round off these numbers to integers (3 and 4).

The empirical formula is Fe₃O₄.

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