Relative molecular mass of ethene (C₂H₄)

Steps:

• Identify molecular formula of ethene as C₂H₄.
• Recall relative molecular mass (Mᵣ) is sum of relative atomic masses (Aᵣ) of all atoms.
• Calculate: 2 × Aᵣ(C) + 4 × Aᵣ(H).
• Verify: Aᵣ(C) = 12, Aᵣ(H) = 1, so Mᵣ = 2(12) + 4(1) = 28.

Why C is correct:

• Matches formula for Mᵣ of C₂H₄: twice Aᵣ of C plus four times Aᵣ of H.

Why the others are wrong:

• A defines relative atomic mass (Aᵣ), not molecular mass.
• B incorrectly halves mass of ¹²C in Aᵣ definition.
• D applies to ethane (C₂H₆), not ethene.

Final answer: C