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Q64 (NDA-I/2024) Science & Technology › Basic Science (Physics, Chemistry, Biology) Official Key

The relative atomic mass of boron (which exists in two isotopic forms 10B and 11B) is 10.81. What will be the abundance of 10B and 11B, respectively (consider a sample of 100 atoms) ?

Result
Your answer: —  Â·  Correct: A
Explanation

The relative atomic mass of an element is the weighted average of the masses of its naturally occurring isotopes. To find the abundance, we use the formula:

Average Atomic Mass = [(Mass of Isotope 1 × % Abundance) + (Mass of Isotope 2 × % Abundance)] / 100

Let the abundance of 10B be x% and the abundance of 11B be (100 - x)%.

  • 10.81 = [10x + 11(100 - x)] / 100
  • 1081 = 10x + 1100 - 11x
  • 1081 = 1100 - x
  • x = 1100 - 1081 = 19

Therefore, the abundance of 10B is 19% and the abundance of 11B is 81% (100 - 19). This matches Option A.

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