Chapter 6 · Chemical Composition
6.6Mass Percent Composition of Compounds
10 min · two checks
Predict
A 2.00 g sample of a compound contains 0.80 g of oxygen. What is the mass percent of oxygen?
The idea
- Calculate mass percent from laboratory masses.
- Recognize that mass percent is a property of a pure compound.
Mass percent of an element is the mass of that element in a sample divided by the total mass, times 100%. For a pure compound the result does not depend on the sample size, which is constant composition again. If you analyze 2.00 g or 20.0 g of the same pure compound, you should get the same percents. Percents of all elements should add to 100, within rounding.
This section uses experimental masses. The next one computes the same kind of percent from a formula, with no lab data except the atomic masses. Do not confuse mass percent with the subscript. Water is two-thirds of its atoms hydrogen and only about 11% hydrogen by mass, because oxygen atoms are heavier.
Keep these
- mass % = (mass of element / mass of compound) × 100%.
- For a pure compound, mass percent is independent of sample size.
- Atom percent and mass percent are different comparisons.