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Percent Composition

Percent Composition. Law of Definite Proportions. The law of definite proportions describes that elements in a given compound are always present in the same proportions by mass. Percentage composition.

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Percent Composition

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  1. Percent Composition

  2. Law of Definite Proportions • The law of definite proportions describes that elements in a given compound are always present in the same proportions by mass.

  3. Percentage composition • The Percentage composition of a compound refers to the relative mass of each element in the compound. • The term molecular massdescribes the mass of a molecule. • The termformula massdescribes the mass of the smallest repeating unit of an ionic compound.

  4. Finding Percentage composition from a chemical formula: • Find the mass of the compound, then find the mass of the individual parts (atoms). • Divide the atoms by the compound and multiply by 100%

  5. Ex: H2O 1. H2 = 2 x 1.01 = 2.02 u O = 16 u 2.02 + 16 = 18.02 u (Total molecular mass) 2. Percentage by mass of H in water = 2/18.02 = 0.11 x 100% = 11% Percentage by mass of O in water = 16/18.02 = 0.888 x 100% = 89%

  6. Ex: Finding Percentage Composition from mass • Take the mass of the atom and divide by the total mass of the compound, then multiply by 100%

  7. Ex: Finding Percentage Composition from mass • Mass of a compound is 48.72g, it contains 32.69g of zinc and 16.03g of sulfur, what percent of the compound is Zn and what percent is S? %Zn = (32.69g/ 48.72g) x 100% = 67.10% %S = (16.03g/ 48.72g) x 100% = 32.90%

  8. Try it: • Calculate the mass percentage of nitrogen in each compound: • N2O • Sr(NO3)2 • NH4NO3 • HNO3 • Page 82 #1, 2, 3, 4 (a only)

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