\(X:C_nH_{2n-2}\\ C_nH_{2n-2} +\dfrac{3n-1}{2} O_2 \xrightarrow{t^o} nCO_2 + (n-1)H_2O\\ n_{O_2} = \dfrac{7,84}{22,4} = 0,35(mol)\\ \Rightarrow n_X = \dfrac{0,7}{3n-1}\\ \Rightarrow \dfrac{0,7}{3n-1}(14n-2) = 3,4\\ \Rightarrow n = 5\\ CTPT\ X :C_5H_8\)