Question

ALLEN
Pre-Medical : Chemistry
19
BEGINNER'S BOX-3
A hydrocarbon contain 80%C. The vapour density of compound is 30. Molecular formula of
compound is :-
(1) CH,
(2) CH,
(3) CH2 (4) All of these
Two elements X (Atomic weight - 75) and Y (Atomic weight = 16) combine to give a compound having 75.8%​

Answer 1

A hydrocarbon contains 80% of carbon.

means, mass percentage of C = 80%

then mass percentage of H = 20% .

find percentage atomic mass ratio,

for C ⇒80/12 = 20/3

for H ⇒20/1 = 20

find simple ratio of given numerical value.

C : H = 20/3 : 20 = 1 : 3

so, empirical formula of hydrogen is

as it is given, vapor density of hydrogen is 30.

so, molar mass of hydrogen = 2 × vapor density

= 2 × 30 = 60 g/mol.

now, molecular mass = n × mass of empirical formula

60 = n (12 × 1 + 1 × 3)

n = 4

hence, molecular formula of hydrocarbon is C4H12