What is the molecular formula of a compound that has a molecular mass of 54 and the empirical formula is c2h3

To find the molecular formula from the empirical formula, you need to find a multiple (x) that will give you the molar mass of the compound which in the question is 54 g/mol.

If C₂H₃ is the empirical formula
 molar mass of empirical formula = (12 × 2) + (1 × 3) g/mol
                                                     =  27 g/mol
let x = multiple
let molecular formula = C₂ₓ H₃ₓ

            multiple = molecular mass ÷ empirical mass
                         =  54 g/mol ÷  27 g/mol
                         =  2

If molecular formula = C₂ₓ H₃ₓ

then molecular formula = C₂₍₂₎H₃₍₂₎

                                      = C₄H₆