Relative formula masses using atomic masses and molecular formula

Steps:

• Atomic mass of magnesium (Mg) is 24, so relative formula mass of one mole of solid Mg is 24.
• Gaseous chlorine exists as diatomic Cl₂ molecules, with atomic mass of Cl at 35.5.
• Relative formula mass of Cl₂ is 2 × 35.5 = 71.
• Thus, the values are 24 for Mg and 71 for Cl₂.

Why C is correct:

• Chlorine gas is Cl₂ per the molecular formula, so its relative formula mass is twice the atomic mass of Cl (71), matching the definition of formula mass as sum of atomic masses in the formula unit.

Why the others are wrong:

• A: Incorrectly uses atomic mass of carbon (12) for Mg and atomic mass of nitrogen (14, close to 17) for Cl.
• B: Correct for Mg but uses single Cl atom mass (35.5) instead of Cl₂.
• D: Doubles Mg mass to 48, ignoring it's monatomic, though correct for Cl₂.

Final answer: C