Some Basic Concepts in Chemistry: Chemistry | JEE Main
Number of hydrogen atoms per molecule of a hydrocarbon A having 85.8% carbon is (Given: Molar mass of A=84 g mol−1 )
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The number of hydrogen atoms per molecule of hydrocarbon A is 12.
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StepWorking
01given
A hydrocarbon contains 85.8% C by mass. The molar mass of the hydrocarbon is M=84 g mol−1.
02find
Find the number of hydrogen atoms per molecule of hydrocarbon A.
03strategise
Since hydrocarbon A consists solely of carbon and hydrogen, the percentage of hydrogen is %H=100−85.8=14.2%. The total mass contributed by hydrogen atoms in one mole of hydrocarbon is MH,total=M×100%H. Dividing by the molar mass of hydrogen (1 g mol−1) gives the number of hydrogen atoms per molecule.
04execute
Percentage of hydrogen: 100−85.8=14.2%.
Total mass of hydrogen in one mole: 84×10014.2=11.928 g.
Number of hydrogen atoms per molecule = 111.928≈12.
✓verify
Mass of carbon = 84−11.928=72.072 g. Number of carbon atoms = 72.072/12≈6. The formula is C6H12, with molar mass 6×12+12×1=84 g mol−1, which matches an alkene/cycloalkane. The result is consistent and physically exact.
The number of atoms in a stable molecular formula must be an integer. The fractional part arises from experimental rounding of the percentage (85.8% instead of 85.714%).
Is CH2 a valid empirical formula for a hydrocarbon?
Yes, it represents the general homologous series of alkenes (CnH2n) or cycloalkanes.