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A carbon sample consists of 95% by mole C-12 (which has 6 neutrons) and 5% by mole C-14 (which has 8 neutrons). Find the average number of neutrons per atom in the sample.

1. 12.5
2. 12.1
3. 6.05
4. 6.1

Correct answer: 6.1

Solution

Number of neutrons: C-12 has 12 - 6 = 6 neutrons; C-14 has 14 - 6 = 8 neutrons. Weighted average = 0.95 * 6 + 0.05 * 8 = 5.70 + 0.40 = 6.10.

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