To determine the empirical formula from percent composition, you can convert the percentages into grams and then into moles.

First, assume you have a $100 \, \text{g}$ sample of the compound. This assumption simplifies the calculations since the percent composition is expressed as mass percentages of each element in the compound. For instance, if a compound consists of $40\%$ carbon, $6\%$ hydrogen, and $54\%$ oxygen, you would interpret this as having $40 \, \text{g}$ of carbon, $6 \, \text{g}$ of hydrogen, and $54 \, \text{g}$ of oxygen in your sample.

Next, convert these masses into moles by dividing the mass of each element by its atomic mass, which can be found on the periodic table. For example, the atomic masses are approximately $12.01 \, \text{g/mol}$ for carbon, $1.01 \, \text{g/mol}$ for hydrogen, and $16.00 \, \text{g/mol}$ for oxygen. Using these values, you can calculate the number of moles as follows:

• For carbon:

• For hydrogen:

• For oxygen:

After determining the number of moles for each element, divide each value by the smallest number of moles calculated to find the simplest whole number ratio. In this example, the smallest mole value is approximately $3.33$. Thus, you would divide each mole quantity by $3.33$:

• For carbon:

• For hydrogen:

• For oxygen:

To express these values in the simplest whole number ratio, we can multiply all ratios by $2$ to eliminate the decimal:

• Carbon: $1 \times 2 = 2$
• Hydrogen: $1.78 \times 2 \approx 4$
• Oxygen: $1.01 \times 2 \approx 2$

Thus, the ratio of carbon to hydrogen to oxygen is $2:4:2$.

Therefore, the empirical formula for this compound is:

It is important to note that the empirical formula represents the simplest whole number ratio of the elements in the compound, not the actual number of atoms in a molecule. The molecular formula may be a multiple of the empirical formula, providing more precise information about the actual composition of the compound.