Percent Composition
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A compound's percent composition states each element's mass as a percentage of the compound's total mass.
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A 10.0-g sample of a gas containing 7.5 g carbon and 2.5 g hydrogen has a percent composition of 75% carbon, 25% hydrogen.
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Percent composition describes mass ratios, not atom-count ratios — a heavier element's percentage can exceed its share of the atom count.