Updated: 7/23/2020

Storyboard Text

  • Hey, do you know what percent composition by mass is?
  • Yes! My mom taught me last year. And I know how to calculate it.
  • Percent composition is simply what percent of an element is in a compound. You can solve for it in 4 easy steps. I'll show you an example, phosphorus pentachloride.
  • p: 1 x 30.975g/mol = 30.975 g/molcl; 5 x 35.453g/mol = 177.265 g/mol
  • The first step is to find the molar mass of all the elements in the compound in grams per mol.
  • PCl5: 1 x 30.975 g/mol + 5 x 35.453 g/mol = 208.239 g/mol
  • The next step is to find the compound's molecular mass.
  • P: 30.75/208.239 = 0.1477Cl: 177.265/208.239 = 0.8513
  • The third step is to divide each component's molar mass by the total molecular mass.
  • PCl5:14.77% phosphorus and 85.13% chlorine by mass
  • The last step is to multiply these numbers by 100. Now they're percentages and we're done!