Question

7.5ml of a hydrocarbons gas was exploded with excess of oxygen. on cooling, it was found to have undergone a contraction of 15ml. if the vapour density of hydrocarbons is 14. determine the molecular formula.

Answer 1

chemical reaction of hydrocarbon with oxygen is ...

$C_xH_y+\left(x+\frac{y}{4}\right)O_2\rightarrow xCO_2(g)+\frac{y}{2}H_2O(l)$

here we see that, 1Volume of hydrocarbon gas converts into x volume of carbon dioxide gas.

given, 7.5 ml of hydrocarbons converts into 15ml.

means, x × 7.5ml = 15ml

or, x = 2.

now chemical formula of hydrocarbon is C2Hy.

and molecular mass = 2 × 12 + y = (24+ y) g/mol....(1)

and vapor density of hydrocarbons is 14.

we know,

molecular mass = 2 × vapor density

so, molecular mass = 2 × 14 = 28 g/mol

from equation (1),

24 + y = 28

y = 4

hence, molecular formula is $C_2H_4$