For aspirin, C9H8O4, with molar mass 180.159 g/mol and O = 16.00 g/mol, what is the mass percentage of oxygen?

1. Work with one mole of aspirin, whose molar mass is 180.159 g/mol. 2. Four oxygen atoms per molecule give 4 x 16.00 g/mol = 64.00 g/mol. 3. The oxygen fraction is 64.00 g/mol divided by 180.159 g/mol. 4. Multiplying by 100 gives 35.52 percent oxygen. 5. Checking against carbon at 60.00 percent and hydrogen at 4.476 percent, the three sum to 100.00 percent. 6. The value 8.88 percent uses only one oxygen atom instead of four. 7. The value 44.40 percent uses five oxygen atoms, a miscount of the subscript. _Source: OpenStax Chemistry (CC BY 4.0), Ch 3 "Composition of Substances and Solutions", section 3.2 Determining Empirical and Molecular Formulas_