A gas is 27.29% C and 72.71% O by mass. What is its empirical formula, using C = 12.01 and O = 16.00 g/mol?

1. Take a 100 g sample, so the masses are 27.29 g C and 72.71 g O. 2. Convert carbon to moles: 27.29 g / 12.01 g/mol = 2.272 mol C. 3. Convert oxygen to moles: 72.71 g / 16.00 g/mol = 4.544 mol O. 4. Divide both by the smaller amount: carbon gives 1 and oxygen gives 4.544 / 2.272 = 2. 5. The ratio is one carbon to two oxygen atoms, so the empirical formula is CO2. 6. Answering CO would come from comparing the masses directly instead of the mole amounts. _Source: OpenStax Chemistry (CC BY 4.0), Ch 3 "Composition of Substances and Solutions", section 3.2 Determining Empirical and Molecular Formulas_