Calculate the number of rings present in `C_(18)H_(12)`.

Explanation

Double bond equivalent formula `=[C-(H)/(2)+(N)/(2)+1]`
Where `C=` no. of carbon atoms. `H=` no. of Hydrogen atoms and `N=` no. of nitrogen atoms.
So, in `C_(18)H_(12)`
Double bond equivalent `=18-(12)/(2)+0+1`
`=18-6+1=13`

One ring is equal to one double bond equivalent.
`:.` Here four rings are there, four double bond equivalent are used. So, remaining , `13-4=9`.
Nine double bons are present in the ring
Hence `C_(18)H_(12)` contain four aromatic rings.