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Chapter 7 Notes, part III

Chapter 7 Notes, part III. -% composition -empirical formulas -molecular formulas. In Review. The mole is an SI unit of measure for amount of particles 6.02x10 23 particles = 1 mole 22.4 L = 1 mole Molar mass=1 mole. % Composition.

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Chapter 7 Notes, part III

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  1. Chapter 7 Notes, part III -% composition -empirical formulas -molecular formulas

  2. In Review • The mole is an SI unit of measure for amount of particles • 6.02x1023 particles = 1 mole • 22.4 L = 1 mole • Molar mass=1 mole

  3. % Composition • The percent composition of an element is the relative amount of that element in a compound. • You find the percent composition of an element by dividing the element’s mass by the mass of the entire compound. mass of element x 100 mass of compound

  4. For example: • What is the % composition of carbon dioxide? Mass of oxygen = 2(16.0)=32.0 g Mass of carbon = 12.0 g Mass of carbon dioxide = 44.0 g 12.0 g C 44.0 g CO2 32.0 g O 44.0 g CO2 x100 = 27.3% Carbon x100 = 72.7% Oxygen

  5. Try these… • What is the percent of each element in phosphorus trichloride? • If 20.0 grams Ca react completely with 16.0 grams of S to form a compound, what is the percent of S?

  6. Empirical Formula • The empirical formula gives you the lowest, whole number ratio of elements in the compound • The empirical formula may or may not be the same as the molecular formula

  7. For example: • For carbon dioxide, the molecular formula is CO2, and the empirical formula is CO2. One carbon and two oxygens are the lowest ratio of atoms. • The molecular formula for dinitrogen tetrahydride is N2H4, but the empirical formula is NH2.

  8. What is the empirical formula for: • C6H12O6 • C6H12O2 • N2H2 • CH4

  9. To Find the Empirical Formula from % Composition: • Given the percentages, assume there are 100.0 grams of the compound. • Convert the grams of each element to moles. • Divide by the smaller amount of moles, then manipulate the ratio so that all numbers are whole.

  10. An Example: • What is the empirical formula of a compound that is 25.9% nitrogen and 74.1% oxygen?

  11. More to try… • Calculate the empirical formula of a compound that is 94.1% oxygen, 5.9% hydrogen. • Calculate the empirical formula of a compound that is 79.8% carbon, 20.2% hydrogen.

  12. From there, find it’s molecular formula: • If given the molar mass (how many g/mol of the compound) then you can calculate the molecular formula from the empirical formula. • Take the compound’s empirical formula mass and compare to the molecular mass. The molecular mass will be a multiple of the empirical formula’s mass.

  13. Example: • If the molecular mass of the first example (N2O5) problem is 216 g/mol, then what is the molecular formula for the compound?

  14. Try these: • What is the molecular formula of a compound whose molar mass is 60.0 g and whose empirical formula is CH4N? • What is the molecular formula for a compound whose molar mass is 78 g and whose empirical formula is CH?

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