Final answer:
The percent composition of a compound is calculated by dividing the mass of an element by the molar mass of the compound and multiplying by 100%. The percentages of the elements in a compound always add up to 100%.
Step-by-step explanation:
The percent composition of a compound is the percent by mass of each element in the compound. It is calculated by dividing the mass of an element in one mole of the compound by the molar mass of the compound, then multiplying by 100%.
For example, let's consider water (H₂O). The molar mass of water is 18 g/mol. Hydrogen (H) has a molar mass of 1 g/mol, so the percent composition of hydrogen in water would be (2 g/18 g) × 100% = 11.1%. Oxygen (O) has a molar mass of 16 g/mol, so the percent composition of oxygen in water would be (16 g/18 g) × 100% = 88.9%.
The percentages of the elements in a compound must always add up to 100%.