Which one of the following is the average atomic mass of chlorine atom ?
- A.35.9 u
- B.35.5 u
- C.35.0 u
- D.37.0 u
Correct answer
Explanation
The correct answer is B, 35.5 u. Natural chlorine is a mixture of two isotopes, and their weighted average is about 35.5 u.
Chlorine has two stable isotopes: chlorine-35, which makes up about 75 per cent of natural chlorine, and chlorine-37, about 25 per cent. The average atomic mass is found by weighting each mass by its share: (35 × 75 + 37 × 25) ÷ 100 = 35.5 u. This is why chlorine's atomic mass is not a whole number, and why chemistry books use 35.5 when working out the molar mass of compounds such as common salt, NaCl, which is 23 + 35.5 = 58.5 g per mole.
Option A is wrong because 35.9 u would need a much larger share of chlorine-37 than nature has. Option C is wrong because 35.0 u is the mass of the chlorine-35 isotope alone and ignores chlorine-37. Option D is wrong because 37.0 u is the mass of the heavier isotope alone.
Exam tip: Cl-35 (75%) and Cl-37 (25%) give 35.5 u; NaCl = 58.5 g/mol.
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