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Percent Composition

Learn how to calculate the percentage of elements in compounds by mass through examples and general formulas. Practice on your own to master the concept!

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Percent Composition

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  1. Percent Composition • Vocabulary • Percent • Percent Composition “No. 5” Mark Rothko Russian 1960

  2. prior knowledge • How could you calculate what percentage of the painting is red, what percentage is black, and what percentage is green?

  3. General Formula for Percent

  4. Percent Composition • Chemists often need to know the percentage of each element in a compound by mass. This is called percent composition.

  5. Example 1 To the right are pictures of copper(II) chloride, CuCl2. If a 34.2 g sample of copper(II) chloride contains 16.2 g of copper and 18.0 g of chlorine, what is the percent composition of CuCl2?

  6. Example 1 Continued • First, determine the percent of copper in the compound: • Next, determine the percent of chlorine in the compound:

  7. Example 2 • What is the percent composition of lithium nitrate? • First we need its formula: • LiNO3 • Notice we don’t have any masses in this problem. Any ideas on what we can use to solve for the percent composition?

  8. Example 2 Continued • We’ll use the molar masses of everything rather than the mass! • Li: 6.94 g/mol • N: 14.01 g/mol • O: 16.00 g/mol • LiNO3: 68.95 g/mol

  9. Example 2 Continued 100.0%

  10. Time to Practice on your Own!!!

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