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Boron occurs as B-10 (20% abundance) and B-11 (80% abundance). Calculate its average atomic mass.
A10.2 amu
B10.5 amu
C10.8 amu
D11.0 amu
Answer & Solution
Correct answer: C. 10.8 amu
1. Convert percentages: B-10 is 0.20 and B-11 is 0.80.
2. Average atomic mass $= (10 \times 0.20) + (11 \times 0.80)$.
3. $= 2 + 8.8$.
4. $= 10.8$ amu.
A simple unweighted mean of 10 and 11 would wrongly give 10.5, but the abundances are unequal.
_Source: Samacheer Kalvi (TN SCERT) Class 10 Science, Unit 7 Atoms and Molecules "Average Atomic Mass", p.100_
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