Which statement describes the percent composition calculation?

Which statement describes the percent composition calculation?

• A. Divide all element masses by the molecular mass to compute percent
• B. Add all molar masses and divide by the molecular mass
• C. Divide each element's molar mass by the molecular mass and multiply by 100
• D. Subtract the smallest molar mass from the largest

Answer: (D)

Percent composition is about how much of the compound’s mass comes from each element, expressed as a percent. To get these percentages, you take the mass contributed by each element in one mole of the compound, divide that by the compound’s total molar mass, and multiply by 100. This unitless fraction becomes the percent for that element, and all the elements’ percentages add up to 100%. For example, in water (H2O) the total molar mass is 18.015 g/mol. Hydrogen contributes 2 × 1.008 = 2.016 g, so its percent is (2.016 / 18.015) × 100 ≈ 11.19%. Oxygen contributes 15.999 g, giving (15.999 / 18.015) × 100 ≈ 88.81%. Subtracting the smallest molar mass from the largest has no relation to percent composition, which is why the correct approach is dividing each element’s molar mass by the total molar mass and multiplying by 100.

Percent composition is about how much of the compound’s mass comes from each element, expressed as a percent. To get these percentages, you take the mass contributed by each element in one mole of the compound, divide that by the compound’s total molar mass, and multiply by 100. This unitless fraction becomes the percent for that element, and all the elements’ percentages add up to 100%.

For example, in water (H2O) the total molar mass is 18.015 g/mol. Hydrogen contributes 2 × 1.008 = 2.016 g, so its percent is (2.016 / 18.015) × 100 ≈ 11.19%. Oxygen contributes 15.999 g, giving (15.999 / 18.015) × 100 ≈ 88.81%.

Subtracting the smallest molar mass from the largest has no relation to percent composition, which is why the correct approach is dividing each element’s molar mass by the total molar mass and multiplying by 100.