Question

The table below gives the data that is needed to calculate the average atomic mass of element B.

Isotope Atomic mass (amu) Relative abundance (%)
B-10 10.01 19.91
B-11 11.01 80.09

Based on these data, what is the average atomic mass of element B?
10.01
10.51
10.81
11.01

Answer 1

Convert the percents to fractions (divide by 100) and take the weighted average:

(10.01 × 0.1991) + (11.01 × 0.8009)
= 1.992991 + 8.817909
= 10.8109 ≈ 10.81

Answer: 10.81.