Question

Analysis of hydrocarbon shows that it consist 85.7% carbon by mass. Calculate the molecular formula of compound, if its vapour density is 28.


C4H10


CH2


C2H4


C4H8

Answer 1

Given

A hydrocarbon contains 85.7% Carbon by mass.

To Find

Its molecular formula

Concept

Vapour density of a gas is given by :

$\dfrac{Molar\:Mass}{2}$

Solution

We have ,

Molar mass of the compound = 2 × Vapour density .

=> Hence, molar mass of the compound = 2 × 28 = 56 gram.

Now, it contains 85.7 ℅ Carbon. Therefore, mass of carbon =

$56\: \times\: \dfrac{85.7}{100}\\ \\ = 47.992 \\ \\ = 48\:gram$

Therefore,mass of carbon in 1 mole compound = 48 gram.

Total mass = 56 gram.

Mass of hydrogen = 56 - 48 = 8 gram.

Now, number of moles of carbon in its 48 gram =

$\dfrac{given \:weight}{Molar\:mass}\\ \\ =\: \dfrac{48}{12}\\ \\ = 4$

Number of moles of hydrogen in its 8 gram = 8.

Thus, 1 mole of the hydrocarbon contains 4 moles of carbon and 8 moles hydrogen

$\textsf{Hence,the formuala of the hydrocarbon is C_4H_8}$

$\boxed{\boxed{\green{The\:answer\:is\:C_4H_8}}}$