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Molecular Formulas

Molecular Formulas. Chapter 6, Section 3C. Molecular Formulas. If you know the composition of a compound, either by mass or percent, you can calculate the empirical formula of the compound

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Molecular Formulas

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  1. Molecular Formulas Chapter 6, Section 3C

  2. Molecular Formulas • If you know the composition of a compound, either by mass or percent, you can calculate the empirical formula of the compound • By knowing the molar mass of the compound, you can find the molecular formula for the compound by following a few simple steps • The molecular formula is always a whole number multiple of the empirical formula

  3. Calculating Molecular Formulas • If necessary, calculate the empirical formula of the compound from the data given. Calculate the empirical formula mass. • Divide the given molecular mass by the empirical formula mass. • n = molecular mass empirical formula mass • Multiple each of the subscripts in the empirical formula by n to get the molecular formula

  4. Example 1 A substance is analyzed and found to have the empirical formula of P2O5. The compound has a molar mass of 283.88 g/mol. What is the molecular formula? • Empirical formula is P2O5. Calculate formula mass? • 2 mol of P = 2 x 30.97 g = 61.94 • 5 mol of O = 5 x 16.00 g = 80.00 g • I mole of P2O5 =141.96 g • n= 283.88 g = 2 141.96 g • Multiply whole equation by 2: • Molecular formula is P4O10

  5. Example 2 A gasoline additive has the following percent composition: 71.65% Cl, 24.27% C, and 4.07% H. The molar mass is known to be 98.96 g. Determine the empirical and molecular formula for this compound. • Finding the Empirical Formula: • 71.65 g Cl, 24.27 g C, 4.07 g H • Convert grams to moles • 71.65 g C1 x 1 molCl= 2.021 mole Cl 35.45 g C1 • 4.07 g H x 1 mol H = 4.03 mole H 1.01 g H • 24.27 g C x 1 molC = 2.021 mole N 12.01 g C

  6. Example 2 Cont. • Finding Empirical Formula Cont. • Both Cl and C had the smallest number moles so divide by 2.012 mol. • 2.012 mol Cl/ 2.012 mol = 1 Cl • 4.03 mol H/ 2.012 mol = 2 H • 2.012 mol C/ 2.012 mol = 1 C • The Empirical Formula is: ClH2C

  7. Example 2 Cont • Finding the molecular formula • Calculate the formula mass from the empirical formula • 1 mol of Cl = 1 x 35.34 g = 35.45 g • 2 mol of H = 2 x 1.01 g = 2.02 g • 1 mol of C = 1 x 12.01 g = 12.01 g • 1 mol of ClH2C = 49.48 g • n = 98.96 g = 2 49.48 g • Multiply the whole equation by: 2 • Molecular formula is: Cl2H4C2

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