A hydrocarbon contains $85.71%$ carbon by mass and has a vapour density of $21$ (C = 12, H = 1). Which of the following statements is/are correct?

A $4.2 g$ of the hydrocarbon contains $1.806 \times 1 0^{23}$ carbon atoms (use $N_{A} = 6.02 \times 1 0^{23} mo l^{- 1}$ ).
BThe molecular formula of the hydrocarbon is $C X_{3} H X_{6}$ .
CThe molecular formula of the hydrocarbon is $C X_{2} H X_{4}$ .
DThe empirical formula of the hydrocarbon is $CH X_{2}$ .

Answer: A, B, D

C : H

= 7.14 : 14.29 = 1 : 2.0

, so the empirical formula is

CH X_{2}

(mass 14).

M = 2 \times 21 = 42

n = 42/14 = 3

, so the formula is

C X_{3} H X_{6}

4.2

g is

0.10

mol, containing

0.30

mol C atoms

= 1.806 \times 1 0^{23}