Number of moles = [tex] \frac{mass (g)}{Molar mass} [/tex] so to calculate molar mass (Molecular weight of compound): Molecular weight = [tex] \frac{mass (g)}{number of moles} [/tex] = [tex] \frac{12.8 g}{0.256 mole} [/tex] = 50 g / mole Now we calculate molecular weight of each compound in choices: a) C₂H₄O = 44 b) CO₂ = 44 c) CH₃Cl = 50.4 ALMOST 50 so this is the correct answer d) C₂H₆ = 30