The percent mass of carbon is found by dividing the total mass of the carbon in the compound by the total mass of the compound itself. The mass of the compound is found by adding the molecular masses of the elements. The mas of the compund is 60 grams/mole. There are two Carbon atoms in the compound and each Carbon atom weighs 12 grams/mole, so two Carbon atoms weigh 24 grams/mole. The percent by mass of Carbon is (24grams/mole) divided by (60 grams/mole) times 100 which is equal to 40%. The correct answer is 40%.
