Q:

What is the difference between an empirical formula and a molecular formula?

A:

The molecular formula describes the number and type of atoms in a single molecule of a substance, while the empirical formula is a simplified version that represents the ratios of the substance's atoms. Since the molecular formula is a multiple of the empirical formula, it can be determined if the ratio between them is found.

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The ratio is given by dividing the actual molar mass of the compound to its empirical mass. The subscripts in the empirical formula are multiplied by the ratio in order to determine the molecular formula. The molecular formula for glucose, for example, is six carbon and oxygen atoms for 12 hydrogen atoms, while the empirical formula is one carbon and oxygen atom for every two hydrogen atoms. The ratio between them is six, and if the amount of each atom in the empirical formula is multiplied by six, the molecular formula is found.

The empirical formula is also found by using a compound's percent composition to determine the amount of moles of the individual atoms. The moles of each atom are compared to each other by dividing them all by the smallest value. All numbers that are close to a whole number are rounded to the nearest integer, and all fractions are eliminated through multiplication leaving only integers behind, which make up the empirical formula.

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