Q.The natural isotopic abundance of 10B is 19.60% and 11B is 80.40 %. The exact isotopic masses are 10.13 and 11.009 respectively. Calculate the average atomic mass of boron (Ans. :10.81)

Step 1. Average atomic mass = [(mass of 10B)(% abundance of 10B) + (mass of 11B)(% abundance of 11B)] / 100.

Step 2. Using the standard isotopic mass of boron-10, 10.01 u (see the fidelity note below), together with 11B's mass 11.009 u and the given abundances 19.60% and 80.40%: [(10.01)(19.60) + (11.009)(80.40)] / 100 = (196.196 + 885.12) / 100 = 1081.32/100 = 10.81 u. …

Apply the weighted-average formula using each isotope's mass multiplied by its pe …

• Using the isotopic mass exactly as printed in the source text (10.13 u) — this does NOT reproduce the book's own stated answer of 10.81; it instead gives about 10.84 u, a clear sign the printed '10.13' …