We are asked to calculate for the percent by mass (mass %) of O in carbon dioxide (CO₂) and to do that we’re going to use the mass percent formula shown below:

$\boxed{\mathrm{mass} \mathrm{percent} = \frac{\mathrm{mass} \mathrm{of} \mathrm{element}}{\mathrm{total} \mathrm{mass} \mathrm{of} \mathrm{compound}}\times 100}$

For the percent by mass O, we’re going to rewrite the equation as: