Lecture 10/15/99, Chapter 4, Section 6

Elemental Anlaysis

1. 1 cup of Coffee has about 50 mg caffeine, (C₈N₄O₂H₁₀),
   1. How many moles of caffeine, C, N, O, and H
   2. How many atoms/molecules of caffeine, C, N, O, and H
   3. What is the percent elemental composition of caffeine
   4. If 89.6475 grams of caffeine is analyzed, how much C, N, O, and H
2. A confiscated white substance, suspected of being cocaine, was purified by a forensic chemist and subjected to elemental analysis. Combustion of a 50.86-mg sample yielded 150.0 mg CO₂ and 46.05 mg H₂O. Analysis for nitrogen showed that the compound contained 9.39% N by mass. The formula of cocaine is C₁₇H₂₁NO₄. Can the forensic chemist conclude that the suspected compound is cocaine?
   1. 150.0 mg CO₂ (this is the carbon from the sample)
      1. 0.1500 g CO₂ at 44 g/mol = 3.41*10^-3 moles CO₂
      2. 1 mole CO₂ contains 1 mole C, so 3.41*10^-3 moles C
      3. 3.41*10^-3 moles * 12.01 g/mol = 4.09*10^-2 g or 40.9 mg C
   2. 46.05 mg H₂O (this is the Hydrogen from the sample)
      1. 0.04605 g H₂O at 18.01 g/mol = 2.56*10^-3 moles H₂O
      2. 1 mole H₂O contains 2 moles H, so sample has
      3. 2.56*10^-3 * 2 = 5.11*10^-3 moles H
      4. 5.11*10^-3 moles * 1.008 g/mol = 5.15*10^-3 grams or 5.15 mg H
   3. The nitrogen analysis gave 9.39% by mass so that
      1. N is 0.0939 * 50.86 mg = 4.78 mg
      2. 0.00478 g * 14.01 g/mol = 3.41*10^-4 mol
   4. The total of C, H, and N = 50.8 mg so the O is the difference
      1. 50.86 - 50.83 mg = 0 mg O
   5. Mole ratios:
      1. C 3.41*10^-3 10
      2. H 5.11*10^-3 15
      3. N 3.41*10^-4 1
      4. O 0 0
   6. Does not support that the compound is cocaine.
3. Derivation of Formulas, Percent Composition, calculate % each, moles each, and mole ratio ( MgSO₄*(H₂O)₇)
   1. 50.00 g sample, heated to release water, weight to 24.43 g
   2. Mg analysis of residue, 4.932 g
   3. S analysis of residue, 6.507 g
   4. O analysis of residue, 12.99 g