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Understanding the Law of Definite Proportions and Percentage Composition in Compounds

The Law of Definite Proportions states that elements within a compound are always present in the same mass ratio. Percentage composition refers to the relative mass of each element in a compound, calculated by dividing the mass of each element by the total mass. For example, in water (H2O), we find the molecular mass (18.02 u) and calculate the mass percentage of hydrogen (11%) and oxygen (89%). This method can also be applied using the mass of the individual elements within a given mass of the compound.

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Understanding the Law of Definite Proportions and Percentage Composition in Compounds

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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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