객관식
What is the mass percent of hydrogen in methane (CH_4)?
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3. Chemical Reactions
Mass Percent
객관식
Caffeine has the molecular formula C8H10N4O2. What is the mass percent of carbon (C) in caffeine?
A
32.0%
B
18.6%
C
61.5%
D
49.5%
검증된 단계별 안내1
Identify the molecular formula of caffeine: C8H10N4O2. This tells us there are 8 carbon atoms, 10 hydrogen atoms, 4 nitrogen atoms, and 2 oxygen atoms in one molecule of caffeine.
Find the atomic masses of each element involved (from the periodic table): Carbon (C) ≈ 12.0 g/mol, Hydrogen (H) ≈ 1.0 g/mol, Nitrogen (N) ≈ 14.0 g/mol, Oxygen (O) ≈ 16.0 g/mol.
Calculate the total molar mass of caffeine by multiplying the number of atoms of each element by their atomic masses and summing all contributions: \(\text{Molar mass} = (8 \times 12.0) + (10 \times 1.0) + (4 \times 14.0) + (2 \times 16.0)\).
Calculate the total mass of carbon in one mole of caffeine by multiplying the number of carbon atoms by the atomic mass of carbon: \(\text{Mass of C} = 8 \times 12.0\).
Calculate the mass percent of carbon by dividing the total mass of carbon by the molar mass of caffeine and multiplying by 100%: \(\text{Mass \% C} = \left( \frac{\text{Mass of C}}{\text{Molar mass of caffeine}} \right) \times 100\%\).
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