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

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

For carbon atom: = ( 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 (

Now, check for unsaturation:

Degree of unsaturation  =

Substitute the value of x and y,

Degree of unsaturation  =

=

= implies one ring and three double bonds.

Thus, formula comes out to be i.e. benzene ring.

1.solid

2.carbon dioxide

3.helium