Question

An element consists of two isotopes. The abundance of one isotope is 95.72% and its atomic mass is 114.9041 u. The atomic mass of the second isotope is 112.9043 u. What is the average atomic mass of the element?
a. 118.9 u
b. 113.0 u
c. 113.9 u
d. 114.8 u

Answer 1

Answer:

d. 114.8 u

Explanation:

The formula for the calculation of the average atomic mass is:

$Average\ atomic\ mass=(\frac {\%\ of\ the\ first\ isotope}{100}\times {Mass\ of\ the\ first\ isotope})+(\frac {\%\ of\ the\ second\ isotope}{100}\times {Mass\ of\ the\ second\ isotope})$

Given that:

For first isotope:

% = 95.72 %

Mass = 114.9041 u

For second isotope:

Since the element has only 2 isotopes, so the percentage of second is 100 - first percentage.

% = 100 %  - 95.72 %  = 4.28 %

Mass = 112.9043 u

Average Mass = ?

Thus,  

$Average\ atomic\ mass=\frac {95.72}{100}\times {114.9041}+\frac {4.28}{100}\times {112.9043}\ u$

Average atomic mass = 114.8 u