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Molar Mass and Percent Composition in Compounds

Learn to calculate molar mass, convert between moles and mass, and determine the percentage composition of compounds. Understand the law of constant composition and how to find mass percent of elements in a compound.

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Molar Mass and Percent Composition in Compounds

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  1. Objectives • To understand the definition of molar mass • To learn to convert between moles and mass • To learn to calculate the mass percent of an element in a compound

  2. A. Molar Mass • A compound is a collection of atoms bound together. • The molar mass of a compound is obtained by summing the masses of the component atoms.

  3. A. Molar Mass • For compounds containing ions the molar mass is obtained by summing the masses of the component ions.

  4. A. Molar Mass • Calculations Using Molar Mass • Moles of a compound = • Mass of a sample (g) = (moles of sample)(molar mass of compound)

  5. Objective: • Calculate the percent composition of a given compound given the formula or experimental data.

  6. B. Percent Composition of Compounds *Remember  The law of constant composition • Any given compound always contains the same elements in the same proportions by mass For example: Carbon dioxide (CO2) contains: 1- carbon atom 2 – oxygen atoms • Always a 2:1 ratio of oxygen:carbon

  7. B. Percent Composition of Compounds • Compounds always have the same # and type of each atoms and also have the same proportions of each element by mass • Percentage Composition – mass of each element in a compound relative to the total mass of the compound.

  8. For example: The Molar mass of carbon dioxide is… • C 12.011 g/mol • O 15.9994 g/mol • O 15.9994 g/mol • 44.010 g/mol • What is the percentage of carbon in CO2? • What is the percentage of oxygen in CO2?

  9. Finding Percentage Composition from experimental data • A sample of 2.45 g of aluminum oxide decomposes into 1.30 g of aluminum and 1.15 g of oxygen. What is the percentage composition of each element in the compound?

  10. B. Percent Composition of Compounds • Percent composition consists of the mass percent of each element in a compound: Mass percent =

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