What is the percent of carbon by mass in C4H10O?

To find the mass of carbon as a percent of the molecule, you have to first know the mass of the whole molecule. This can be done with the atomic weights of the elements and multiplying them by the number of atoms in a single molecule of hexachlorophene (the given compound).

Carbon: 12.0 g/mol × 13 = 156

Hydrogen: 1.0 g/mol × 6 = 6.0

Chlorine: 35.5 g/mol × 6 = 213

Oxygen: 16.0 g/mol × 2 = 32.0

C13H6Cl6O2 = 407 g/mol

(Mass of carbon ÷ total mass) × 100 = % carbon by mass

(156 ÷ 407) × 100 = 38.3% carbon