Weighted Average Atomic Mass

Steps:

• The atomic mass of 35.5Cl is the weighted average of isotope masses and abundances.
• Let x = % of ^{35}Cl; then (100 - x) = % of ^{37}Cl.
• Set up equation: [35x + 37(100 - x)] / 100 = 35.5.
• Solve: 35x + 3700 - 37x = 3550 → -2x = -150 → x = 75% (so 25% ^{37}Cl).

Why D is correct:

• 75% ^{35}Cl and 25% ^{37}Cl yield (35 × 0.75 + 37 × 0.25) = 35.5, matching the given value.

Why the others are wrong:

• A: Yields average of 36.5, higher than 35.5.
• B: Yields average of 36.2, higher than 35.5.
• C: Yields average of 36, higher than 35.5.

Final answer: D