JEE Main · 2023 · 25 Jan 2023 · Shift 2 · Some Basic Concepts of Chemistry

Number of hydrogen atoms per molecule of a hydrocarbon A having 85.8% carbon is ____
(Given : Molar mass of A

= 84 g mol^{- 1}

)

Answer: 12

Solution

With

85.8%

carbon the hydrogen content is

14.2%

. Dividing by the atomic masses, carbon gives

85.8/12 = 7.15

and hydrogen

14.2/1 = 14.2

, a ratio of

1 : 2

, so the empirical formula is

C H_{2}

of mass

14 g mo l^{- 1}

. Since the molar mass is

84 g mo l^{- 1}

, the molecule contains

84/14 = 6

such units, giving

C_{6} H_{12}

. The number of hydrogen atoms per molecule is twelve.

JEE Main · 2023 · 25 Jan 2023 · Shift 2 · Integer answer