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Empirical Formulas

Empirical Formulas. Reading activity . With a partner take turns reading pages 241-246 Discuss the relationships between the following words and create a small word web to show how they are related Molecular Formula Empirical Formula Percent Composition Mole Molar Mass “n”. Definition.

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Empirical Formulas

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  1. Empirical Formulas

  2. Reading activity • With a partner take turns reading pages 241-246 • Discuss the relationships between the following words and create a small word web to show how they are related • Molecular Formula • Empirical Formula • Percent Composition • Mole • Molar Mass • “n”

  3. Definition • Empirical Formulas of a chemical compound is shows the smallest whole number ratio of elements in a compound. • Standard for ionic compounds

  4. Where does it come from? • The term empirical refers to a process of elemental analysis used to determine relative percent composition of a pure chemical substance by element.

  5. Molecular Formula • Molecular Formula identifies the absolute number of atoms of such elements to be found in each discrete molecule of that compound.

  6. Examples

  7. You may be given the following info: • The masses of the elements in a weighed sample of the compound • The mass percents of the element in the compound • Assume a 100-g sample and calculate the masses of each element in that sample

  8. From the information given… • Find the number of moles of each element. (Gram to mole conversion usually) • Find the simplest mole ratio and set that to 1. • Find all other mole ratios. • Apply the mole ratios as subscripts for your simple formula. • If a mole ratio is X 0.5, then multiply ALL mole ratios by 2.

  9. #1 • 0.6884gPb = 3.322*10-3 • 0.2356gCl = 6.6646*10-3 Mass of Pb Mass of Cl

  10. Pb = 1.000 mol Pb Cl: = 2.001 mol Cl ANSWER: PbCl2 Divide by the smallest number

  11. #2 • HINT: Assume 100 grams • Answer: C6NH11O • Answer #3: C3H3O

  12. Molecular Formulas • 71.65 g Cl = 2.021mol Cl • 24.27 g C = 2.021mol C • 4.07 g H = 4.04mol H • Ratio: ClCH2

  13. The molar mass of ClCH2 is 49.48 g (empirical formula mass) • = = 2 ; • thus the Molecular Formula is (ClCH2)2 or Cl2C2H4

  14. Practice Problems • Complete on the back or new page in your notebook • Page 248 • Problems: 1-5, 8 & 10

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