Dual Enrollment Physical Science Midterm Practice Test

Think in terms of mass percent: what portion of the molecule’s mass comes from carbon. In CO2, there is one carbon atom with about 12.01 amu, and two oxygens with 2×16.00 = 32.00 amu. The molar mass of CO2 is roughly 12.01 + 32.00 = 44.01 g/mol. The carbon portion of that mass is 12.01 g per mole. So the percent by mass of carbon is (12.01 / 44.01) × 100 ≈ 27.3%. Using more precise atomic masses gives about 27.29%.