Determining Empirical Formula from Percent Composition

Determining an empirical formula from percent composition means treating each percentage as grams in a 100-g sample, converting each mass to moles, and reducing those mole amounts to a small whole-number ratio.

A gas is 27.29% carbon and 72.71% oxygen. Treating this as a 100-g sample gives 27.29 g carbon and 72.71 g oxygen. Dividing by each element's molar mass gives about 2.273 mol carbon and 4.544 mol oxygen. Dividing both mole amounts by the smaller value, 2.273, gives a ratio of 1 to 2.00, so the empirical formula is CO2.