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: $\frac{78}{12}$ = $6\frac{6}{12}$ ( 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  = $\frac{2x+2-y}{2}$

Substitute the value of x and y,

Degree of unsaturation  = $\frac{2(6)+2-6}{2}$

= $\frac{12-4}{2}$

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

Thus, formula comes out to be $C_{6}H_{6}$ i.e. benzene ring.