1.3 Elemental Composition of Pure Substances and Composition of Mixtures

Percent Composition

The percent composition is the percent by mass of an element in a compound. According to the law of definite proportions, a compound always consists of the same proportion of elements by mass. To find the percent composition of an element, you need to divide the mass of the element by the total mass of the compound. If there are 11.1g of hydrogen in 100g of water, the percent composition of hydrogen is 11.1/100, which is 11.1%. The percent composition can also be found using the formula of the compound. The percent composition of carbon in carbon dioxide is 27.27%. The formula of carbon dioxide is ${CO_2}$. If you divide 12, the molar mass of carbon, by 44 (the molar mass of carbon plus the molar mass of two oxygens), you get 27.27%.