`11.6g` of an organic compound having formula `C_n H_(2n+2)` is burnt in excess of `O_2(g)` initially taken in a `22.41` litre steel vessel. Before reactio0n the gaseous mixture was at `273K` with pressure reading 2 atm. Aftercomplete combustion and loss of considerable amount of heat, the mixture of product and excess of `O_2` had a temperature of `546K and 4.6` atm pressure. The formula of organic compound is :