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Calculating Empirical and Molecular Formulas

Calculating Empirical and Molecular Formulas. A chemical formula in which the ratio of the elements are in the lowest terms is called an empirical formula . For example, the empirical formula for a glucose molecule (C 6 H 12 O 6 ) is CH 2 O. All the subscripts are

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Calculating Empirical and Molecular Formulas

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  1. Calculating Empirical and Molecular Formulas

  2. A chemical formula in which the ratio of the elements are in the lowest terms is called an empirical formula. For example, the empirical formula for a glucose molecule (C6H12O6) is CH2O. All the subscripts are divisible by six. Some formulas, such as the one for carbon dioxide, CO2, are already empirical formulas without being reduced. To find the empirical formula from the molecular formula, you must divide all the subscripts by the greatest common factor. Empirical Formulas

  3. C8H18…..______________ H2O…….______________ H2O2……______________ C4H8……______________ Hg2Cl2…..______________ C4H6….._______________ C3H6…….______________ C7H12…________________ Na2C2O4. .______________ CH3COOH….____________ Determine the empirical formulas for each of the following molecular formulas.

  4. Find mass (or %) of each element. • Find moles of each element. • Divide moles by the smallest # to find subscripts. • When necessary, multiply subscripts by 2, 3, or 4 to get whole #’s. Finding empirical formulas using molecular mass:

  5. 25.9g_ = 1.85 mol N 1.85mol N = 1 N x 2 = N2 14g/mol 1.85 mol 74.1 g_ = 4.63 mol O4.63 mol O = 2.5 O x 2 = O5 16 g/mol 1.85 mol EMPIRICAL FORMULA: N2O5 EXAMPLE: Find the empirical formula for a sample of 25.9% N and 74.1% O.

  6. A substance is 36.1% by weight calcium and 63.9% chlorine. What is the empirical formula of this compound? PRACTICE PROBLEMS

  7. A compound is 43.4%Na, 11.3%C, and 45.3%O. What is the empirical formula for this compound?

  8. A molecular formula is the “true formula” of a compound. The chemical formula for a molecular compound shows the actual number of atoms present in a molecule. MOLECULAR FORMULAS

  9. Find the empirical formula. • Determine the empirical formula mass. • Divide the molecular mass by the empirical formula mass to determine the multiple. • Multiply the empirical formula by the multiple to find the molecular formula. MF mass = n EF mass (EF)n = molecular formula To find the molecular formula from the empirical formula:

  10. C = 1 x 12 = 12 H = 2 x 1 = +2 14g/mol = empirical formula mass 28.1 g/mol = 2 14 g/mol (CH2)2 C2H4 EXAMPLE: The empirical formula for ethylene is CH2. Find the molecular formula if the molecular mass is 28.1g/mol.

  11. Find the molecular formula for a compound with a mass of 78 amu and the empirical formula CH. PRACTICE PROBLEMS

  12. Find the molecular formula for a compound with a mass of 82 amu and the empirical formula C3H5.

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