Which of the following pairs of components have the same molecular mass? [ H = 1.0, C = 12.0, O = 16.0 ]

Analyze the question: We need the pair of compounds with equal molar (molecular) masses, using H = 1, C = 12, O = 16.

Step 1: Write the formulas

• Ethene: C₂H₄
• Ethyne: C₂H₂
• Ethane: C₂H₆
• Carbon(II) oxide: CO
• Carbon(IV) oxide: CO₂

Step 2: Calculate the molecular masses

• Ethene, C₂H₄: (2 × 12) + (4 × 1) = 28
• Ethyne, C₂H₂: (2 × 12) + (2 × 1) = 26
• Ethane, C₂H₆: (2 × 12) + (6 × 1) = 30
• Carbon(II) oxide, CO: 12 + 16 = 28
• Carbon(IV) oxide, CO₂: 12 + (2 × 16) = 44

Step 3: Compare each pair

• A. Ethene (28) and Ethyne (26): not equal
• B. Ethane (30) and Ethene (28): not equal
• C. CO (28) and Ethene (28): equal
• D. CO₂ (44) and Ethyne (26): not equal

Answer: C. Carbon(II) oxide and ethene both have a molecular mass of 28.

Tip: Memorize the common "28 pair" (CO, C₂H₄, and also N₂). Compounds with the same molecular mass but different formulas often appear in exams. For hydrocarbons, count only carbon (×12) and hydrogen (×1) quickly, and remember that the Roman numeral in names like carbon(II) oxide tells you the oxidation state of carbon, hence CO versus CO₂.