What is the molecular formula for ethene?

What is the molecular formula for ethene?

• A. C2H4
• B. C3H6
• C. C2H6
• D. C4H8

Answer: (A)

Ethene is the simplest alkene, containing two carbon atoms joined by a carbon–carbon double bond, with two hydrogens attached to each carbon. This structure gives the formula C2H4: two carbons and four hydrogens in total. The double bond reduces the number of hydrogens compared with the saturated hydrocarbon with the same number of carbons (ethane would be C2H6), so ethene ends up with four hydrogens. Other possibilities would not fit because they either have the wrong number of carbons or the wrong hydrogen count for two carbons. For example, a formula with three carbons would be a different hydrocarbon (propene or cyclopropane), while a formula with two carbons and six hydrogens corresponds to ethane, a single-bonded saturated compound, and a formula with four carbons and eight hydrogens corresponds to butenes or cyclobutane. Hence, C2H4 is the correct molecular formula.

Ethene is the simplest alkene, containing two carbon atoms joined by a carbon–carbon double bond, with two hydrogens attached to each carbon. This structure gives the formula C2H4: two carbons and four hydrogens in total. The double bond reduces the number of hydrogens compared with the saturated hydrocarbon with the same number of carbons (ethane would be C2H6), so ethene ends up with four hydrogens.

Other possibilities would not fit because they either have the wrong number of carbons or the wrong hydrogen count for two carbons. For example, a formula with three carbons would be a different hydrocarbon (propene or cyclopropane), while a formula with two carbons and six hydrogens corresponds to ethane, a single-bonded saturated compound, and a formula with four carbons and eight hydrogens corresponds to butenes or cyclobutane. Hence, C2H4 is the correct molecular formula.