Question

What is the molecular formula of a hydrocarbon with m+ = 78?

Answer 1

A molecular formula represents the exact number of atoms present for each element in the compound.

For carbon atom:
`(78)/(12)` =
`6(6)/(12)` ( as molar mass of carbon is 12 g/mol)

Now, 6 carbon atoms are present and rest are hydrogen atoms i.e. 6

Thus, formula becomes
`C_(6)H_(6)` (
`C_(x)H_(y))`

Now, check for unsaturation:

Degree of unsaturation =
`(2x+2-y)/(2)`

Substitute the value of x and y,

Degree of unsaturation =
`(2(6)+2-6)/(2)`

=
`(12-4)/(2)`

=
`4` implies one ring and three double bonds.

Thus, formula comes out to be
`C_(6)H_(6)` i.e. benzene ring.