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Atomic weight of Boron is 10.81 and it has two isotopes B^10 and B^11. Then the ratio of B^10 : B^11 in nature would be

1. 19 : 81
2. 10 : 11
3. 15 : 16
4. 81 : 19

Correct answer: 19 : 81

Solution

The atomic weight of Boron is a weighted average of its isotopes. Let the ratio of B^10 to B^11 be x : y. Using the equation 10x + 11y = 10.81(x + y), solving gives x : y = 19 : 81.

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