Question

The atomic masses of two isotopes of an element are 16 and 17 respectively. Their percentage abundances are 25% and 60% respectively and the average atomic mass of the element is 16.90. If the element has 3 isotopes, calculate the atomic mass of the third isotope.

• A. 17
• B. 18
• C. 20
• D. 15

Answer 1

Answer: (B)

18

Let the atomic mass of the third isotope be A.
The average atomic mass of the element
$16.90=\frac{[25\times 16]+[60\times 17]+[15\times A]}{25+60+15}$
$1690=[25\times 16]+[60\times 17]+[15\times A]$
$1690=1420+15A$
$15A=270$
$A=18$
The atomic mass of the third isotope is 18.