To determine the empirical formula from percentage composition, which sequence is correct?

• A. Convert percentages to masses, convert to moles using relative atomic masses, divide by smallest mole, then use the simplest whole-number ratio to form the empirical formula.
• B. Divide percentages by atomic masses, then multiply until whole numbers.
• C. Use percentages directly as mole ratios.
• D. Determine molecular formula first.

Answer: (A)

When you’re given percentage composition and need the empirical formula, the aim is to get the simplest whole-number ratio of atoms. A practical way is to imagine a 100 g sample, so each percentage becomes a mass in grams. Convert those masses to moles by dividing each mass by that element’s relative atomic mass. Now compare the mole amounts by dividing all of them by the smallest number of moles obtained. This yields the simplest whole-number ratio of atoms. If any values are fractional, multiply all the ratios by the same factor to clear the fractions, giving the empirical formula.

The other approaches fall short because one step is missing or incorrect. Dividing percentages by atomic masses and then just multiplying until you get whole numbers skips the essential normalizing step of dividing by the smallest mole count to obtain the simplest ratio. Using percentages directly as mole ratios ignores the fact that different elements have different molar masses, so percentages aren’t directly proportional to moles. Determining the molecular formula first isn’t appropriate because you first need the empirical formula (the simplest ratio) and, typically, the molar mass, to scale up to the actual molecule.