Question

Calculate the average atomic mass of B. The isotopes and abundances are 10B, 19.80% and 11B, 80.20%. Round answer to 3 significant digits.
A. 1.98 amu
B. 10.8 amu
C. 10 amu
D. 8.82 amu

Answer 1

Given,

The two isotopes of B are 10B and 11B

% abundance of 10B = 19.80

% abundance of 11B = 80.20

Average atomic mass of B

= $\frac{(mass of 10B)(abundance of 10B) + (mass of 11B)(abundance of 11B)}{100}$

= $\frac{(10)(19.80) + (11)(80.20)}{100}$

= $\frac{198 + 882.2}{100}$

= $\frac{1080.2}{100}$

= 10.802

Therefore, the average atomic mass of B is 10.802 u

Answer 2

Answer: Correct option is B.

Explanation: In this question, we are given two isotopes of Boron.

Mass of isotope $B^{10}=10amu$

Mass of isotope $B^{11}=11amu$

Fractional abundance can be calculated as:

$\text{Fractional abundance}=\frac{\% abundance}{100}$

Fractional abundance of isotope $B^{10}=\frac{19.80}{100}=0.198$

Fractional abundance of isotope $B^{11}=\frac{80.20}{100}=0.802$

To calculate average atomic mass of an element, we use:

$\text{Average Atomic Mass}=\sum_{i=1}^{n}(\text{Fractional abundance})_i\times (\text{Mass number})_i$

Now, putting the values of abundances and mass number of 2 isotopes in above equation, we get:

$\text{Average Atomic mass of B}=(0.198\times 10amu)+(0.802\times 11amu)$

Average Atomic Mass of B = 10.802 amu

Rounding it off to 3 significant figures, we get

Average atomic mass of B = 10.8 amu.