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Organic Chemistry

Organic Chemistry. Review Information for Unit 1 Determining Empirical Formulas Determining Molecular Formulas. Calculating Empirical & Molecular Formulas. The empirical formula for an unknown compound can be determined using quantitative elemental analysis.

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Organic Chemistry

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  1. Organic Chemistry Review Information for Unit 1 Determining Empirical Formulas Determining Molecular Formulas

  2. Calculating Empirical & Molecular Formulas • The empirical formula for an unknown compound can be determined using quantitative elemental analysis. • Often, the % composition for all elements except oxygen is reported • Example: 40.0% C and 6.67% H • Remainder (unless it adds up to very close to 100%) is assumed to be oxygen • In previous example: 53.3% O

  3. Calculating Empirical & Molecular Formulas • Calculate empirical formula by: • % to mass • mass to mole • divide by smallest • multiply ‘til whole

  4. Calculating Empirical & Molecular Formulas Example: Calculate the empirical formula for a compound containing 81.8% C and 18.2% H. Steps 1 & 2: % to Mass; Mass to Moles

  5. Calculating Empirical & Molecular Formulas Step 3: Divide by Smallest C: 6.82 = 1.00 = 1 6.82 H: 18.0 = 2.64 Step 4: Multiply ‘til Whole Since CH2.64 doesn’t make any sense, you must multiply to get a whole number ratio. 2.64 ~ 2.67 ~ 2 2/3 or 8/3. So multiply both numbers by 3. C3H8

  6. Calculating Empirical & Molecular Formulas • Molecular formulas are always some whole number multiple of the empirical formula: C2H4O (acetic acid) CH2O C6H12O6 (glucose) X 2 X 6

  7. Calculating Empirical & Molecular Formulas • Molecular weight is the same whole number multiple of the empirical formula’s formula weight (FW). C2H4O (acetic acid) CH2O C6H12O6 (glucose) X 2 MW = 30.0 amu x 2 = 60.0 amu FW = 30.0 amu X 6 MW = 30.0 amu x 6 = 180.0 amu

  8. Calculating Empirical & Molecular Formulas • Molecular weights can be determined experimentally in several ways: • colligative properties • ideal gas law • mass spectrometry • To determine the molecular formula for an unknown: • calculate the empirical formula • calculate ratio of MW/empirical FW • multiply subscripts of empirical formula by previous ratio

  9. Calculating Empirical & Molecular Formulas Example:Determine the molecular formula for a compound that contains 39.8 % C, 3.32% H, and 39.2% Cl if its molecular weight is about 181 amu. Step 1: Find Empirical Formula

  10. Calculating Empirical & Molecular Formulas Empirical Formula = C3H3ClO Step 2: Find MW/FW (empirical) MW = 181 (given) FW (empirical) = 3(12.0) + 3 (1.01) + 35.5 + 16.0 = 90.5 MW = 181 = 2 FW 90.5 Step 3: Multiply subscripts Molecular Formula = C(3x2)H(3 x 2)Cl(1x2)O(1x2) = C6H6Cl2O2

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