Question

Which compound has the same empirical and molecular formula ethyne ethene ethane methane?

Answer 1

Empirical formula is the simplest ratio of whole numbers of components in a compound 
molecular formula is the actual ratio of components in a compound .
the molecular formula for the compounds given are as follows
ethyne  - C₂H₂
ethene - C₂H₄
ethane - C₂H₆
methane - CH₄
the actual ratios of the elements                 simplified ratio
                     C : H                                           C : H
ethyne            2:2                                             1:1
ethene            2:4                                             1:2
ethane            2:6                                             1:3
methane         1:4                                             1:4
the only compound where the actual ratio is equal to the simplified ratio is methane 
therefore in methane molecular formula CH₄ is the same as empirical formula CH₄