Percentage Composition by Mass

- Definition: All compounds are formed when two or more elements chemically combine.  

- Since this involves a chemical change, the product must have a definite formula.  

- The formula of a compound is defined by the elements making it up, combined in specific ratios or percentages.  

Formula:  

% by Mass = Mass of element / Molar mass of compound x 100  

Example:  

For water (H₂O):  

- Mass of H = 2 × 1 = 2 g  

- Mass of O = 16 g  

- Molar mass of H₂O = 18 g  

- % by mass of H = (2 ÷ 18) × 100 = 11.1%  

- % by mass of O = (16 ÷ 18) × 100 = 88.9%