Question

Carbon exists in nature as three isotopes. Given the atomic masses and relative abundance of the isotopes, what is the average atomic mass of carbon?
A. 12.000 amu
B.12.011 amu
C.13.002 amu
D.13.003 amu
E.14.003 amu

Answer 1

The correct answer is B. 12.011 amu

Answer 2

Answer : The correct option is, (B) 12.011 amu

Explanation :

Average atomic mass of an element is defined as the sum of masses of each isotope each multiplied by their natural fractional abundance.

Formula used to calculate average atomic mass follows:

$\text{Average atomic mass }=\sum_{i=1}^n\text{(Atomic mass of an isotopes)}_i\times \text{(Fractional abundance})_i$

We are assuming that:

Mass of isotope C-12 = 12.000 amu

% abundance of isotope C-12= 98.93% = 0.9893

Mass of isotope C-13 = 13.003 amu

% abundance of isotope C-13 = 1.07% = 0.0107

Mass of isotope C-14= 14.003 amu

% abundance of isotope C-14 = 0.0000000001% =  0.000000000001 = $1.0\times 10^{-12}$

In this case, the abundance of C-14 is very low. So, their influence is negligible.

Now put all the given values in the above formula, we get:

$\text{ Average atomic mass of an element}=\sum[(12.000\times0.9893)+(13.003\times 0.0107)+(14.003\times 1.0\times 10^{-12})]$

$\text{ Average atomic mass of an element}=12.011amu$

Therefore, the average atomic mass of carbon is, 12.011 amu