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

Answer: Correct option is B.

Step-by-step 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}=(\% abundance)/(100)`

Fractional abundance of isotope
`B^(10)=(19.80)/(100)=0.198`

Fractional abundance of isotope
`B^(11)=(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* (\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* 10amu)+(0.802* 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.

Answer 2

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

=
`((mass of 10B)(abundance of 10B) + (mass of 11B)(abundance of 11B))/(100)`

=
`((10)(19.80) + (11)(80.20))/(100)`

=
`(198 + 882.2)/(100)`

=
`(1080.2)/(100)`

= 10.802

Therefore, the average atomic mass of B is 10.802 u