Question

An element has two naturally-occurring isotopes. The mass numbers of these isotopes are 113 amu and 115 amu, with natural abundances of 5% and 95%, respectively. Calculate its average atomic mass. Report your answer to 1 decimal place.

Answer 1

Answer: Its average atomic mass is 114.9 amu

Explanation:

Mass of isotope 1 = 113 amu

% abundance of isotope 1 = 5% = $\frac{5}{100}=0.05$

Mass of isotope 2 = 115 amu

% abundance of isotope 2 = 95% = $\frac{95}{100}=0.95$

Formula used for average atomic mass of an element :

$\text{ Average atomic mass of an element}=\sum(\text{atomic mass of an isotopes}\times {{\text { fractional abundance}})$

$A=\sum[(113\times 0.05)+(115\times 0.95)]$

$A=114.9amu$

Thus its average atomic mass is 114.9 amu