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Understanding Empirical and Molecular Formulas in Chemistry

This section covers the concepts of empirical and molecular formulas, emphasizing their importance in chemistry. An empirical formula indicates the relative number of atoms of each element in a compound, exemplified by H₂O. The mole concept allows for calculations needed to derive empirical formulas, illustrated through real-world examples, such as determining the empirical formula for a compound containing mercury and chlorine, leading to HgCl₂. Additionally, it outlines how to find molecular formulas from empirical formulas using the whole number multiple method.

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Understanding Empirical and Molecular Formulas in Chemistry

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  1. Empirical and Molecular Formulas Section 3.5

  2. Empirical Formulas • Tells us relative number of atoms of each element it contains • Example: H2O: 2 atoms of H per 1 atom of O • ALSO: H2O: 2 mol of H per 1 mol of O • Mole concept allows us to calculate the empirical formula

  3. Example • Compound is 73.9% Hg and 26.1% Cl. Find empirical formula. • Assume 100 grams 73.9 g Hg x (1 mol Hg) = 0.368 mol Hg 200.6 g Hg 26.1 g Cl x (1 mol Cl) = 0.735 mol Cl 35.5 g Cl mol Cl= 0.735 mol Cl= 1.99 mol Cl = 2 = HgCl2 mol Hg 0.368 mol Hg = 1 mol 1

  4. Sample Exercise 3.13 p. 97 • Ascorbic acid: 40.92% C, 4.58% H, and 54.50 %O • Always divide the larger numbers (C and H) of moles by the smallest number of moles (O)

  5. Molecular Formula • Have to find Empirical Formula first!!! • Whole number multiple = molecular weight empirical formula weight • Multiple each subscript of the empirical formula by the whole number multiple

  6. Back to Sample Exercise 3.13 • Empirical formula = C3H4O3 • 1st, find empirical formula weight: • 3(12.0) + 4(1.0) + 3(16.0) = 88.0 amu • Given: experimentally determined molecular weight = 264 amu • W.N.M.= molecular weight = 264 = 3 empirical formula weight 88 • Molecular Formula = C9H12O9

  7. Homework • 3.44- 3.50 even only on page 113-114

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