Calculating relative atomic mass from isotopic data

Steps:

• Identify isotopes and abundances in the sample (e.g., Pb-204 at 1.4%, Pb-206 at 24.1%, Pb-207 at 22.1%, Pb-208 at 52.4%).
• Multiply each isotope's mass by its fractional abundance.
• Sum the products to get the weighted average mass.
• Round to appropriate significant figures for the relative atomic mass.

Why B is correct:

• 207.25 matches the standard weighted average from lead's natural isotopic abundances, per IUPAC definition of relative atomic mass as abundance-weighted mean.

Why the others are wrong:

• A underestimates by ignoring heavier isotopes like Pb-208.
• C overestimates slightly, possibly from rounding error in abundances.
• D is too high, akin to pure Pb-208 mass without lighter isotope contributions.

Final answer: B