Question

Caffeine (C 8 H 10 N 4 O 2 ), is a stimulant found in coffee, tea, colas and energy drinks. It is

used to restore mental alertness or wakefulness during fatigue or drowsiness. Caffeine is

also found in some headache and migraine medications. Solve for its percent

composition.

Answer 1

Solution:

Atomic mass of elements:

A_r(C) = 12.0107 amu

A_r(H) = 1.00784 amu

A_r(N) = 14.0067 amu

A_r(O) = 15.999 amu

Calculate the molar mass of caffeine (C₈H₁₀N₄O₂):

M(C₈H₁₀N₄O₂) = 8×A_r(C) + 10×A_r(H) + 4×A_r(N) + 2×A_r(O)

M(C₈H₁₀N₄O₂) = 8×12.0107 + 10×1.00784 + 4×14.0067 + 2×15.999 = 194.1888 (g/mol)

Calculate the percent composition of caffeine (C₈H₁₀N₄O₂):

w(C) = [8×A_r(C) / M(C₈H₁₀N₄O₂)] × 100%

w(C) = [8×12.0107 / 194.1888] × 100% = 49.48%

w(C) = 49.48%

w(H) = [10×A_r(H) / M(C₈H₁₀N₄O₂)] × 100%

w(H) = [10×1.00784 / 194.1888] × 100% = 5.19%

w(H) = 5.19%

w(N) = [4×A_r(N) / M(C₈H₁₀N₄O₂)] × 100%

w(N) = [4×14.0067 / 194.1888] × 100% = 28.85%

w(N) = 28.85%

w(O) = [2×A_r(O) / M(C₈H₁₀N₄O₂)] × 100%

w(O) = [2×15.999 / 194.1888] × 100% = 16.48%

w(O) = 16.48%

Answer:

Caffeine contains 49.48% Carbon, 5.19% Hydrogen, 28.85% nitrogen, 16.48% oxygen.