Weighted average of isotope masses

Steps:

• Determine percentage of 32S: 100% - 4.2% - 2.8% = 93%.
• Calculate contributions: (93 × 32) + (4.2 × 34) + (2.8 × 36) = 2976 + 142.8 + 100.8 = 3219.6.
• Divide by 100 to get average mass: 3219.6 / 100 = 32.196.
• Round to one decimal place: 32.2.

Why B is correct:

• Relative atomic mass is the weighted average of isotope masses by abundance, yielding 32.2 here.

Why the others are wrong:

• A: Underestimates heavier isotope contributions by about 0.1.
• C: Ignores dominant 32S, averaging only heavier isotopes.
• D: Overweights heavier isotopes, exceeding calculated value.

Final answer: B