Caffeine has an elemental analysis of 49.48% carbon, 5.190% hydrogen, 16.47% oxygen, and 28.85% nitrogen. It has a molar mass of 194.19 g/mol. What is the molecular formula of caffeine? A)C₄H₅N₂O B)C₈H₁₀N₄O₂ C)C₁₀H₁₄N₂O₂ D)C₆H₆NO₂ E)C₅H₇NO

Question

Quizplus · Accepted Answer

1) Find the empirical formula:
- Assume 100g sample, then it contains:
49.48g C / 12.01 g/mol = 4.120 mol C
5.190g H / 1.008 g/mol = 5.150 mol H
16.47g O / 16.00 g/mol = 1.029 mol O
28.85g N / 14.01 g/mol = 2.064 mol N
- Divide by smallest value (1.029 mol):
C4.120H5.150N2.064O1.029
- Multiply by 3 to simplify the formula:
C12.36H15.45N6.192O3.087
2) Calculate the empirical formula mass:
12.36 g/mol (C) + 15.45 g/mol (H) + 6.192 g/mol (N) + 3.087 g/mol (O) = 37.08 g/mol
3) Calculate the factor to obtain the molecular formula mass:
194.19 g/mol / 37.08 g/mol = 5.238
4) Multiply the empirical formula by the factor:
C12.36 x 5.238 = 64.7428 ≈ 65
H15.45 x 5.238 = 80.9871 ≈ 81
N6.192 x 5.238 = 32.449 ~ 32
O3.087 x 5.238 = 16.159 ~ 16
- Molecular formula: C65H81N32O16
- Simplify the formula by 2:
Caffeine: C8H10N4O2

Caffeine Has an Elemental Analysis of 49