Question

Natural chlorine consists of ${}^{35}$Cl and ${}^{37}$Cl . Their atomic masses are $34.97$ amu and $36.97$ amu, respectively. If the average atomic mass of chlorine is $35.5$ amu , the abundance of ${}^{35}$Cl is :

• A. $24.0$%
• B. $35.5$%
• C. $50.0$%
• D. $75.5$%

Answer 1

Answer: (D)

$75.5$%

Let the natural abundance of ${}^{35}Cl$ isotope be $x$% and that of ${}^{37}Cl$ be $(100-x)$%

Average atomic mass$=\frac{\text{Atomic mass of}Cl-35x+\text{Atomic mass of}Cl-37\times (100-x)}{100}$

$35.5=\frac{34.97x+36.97(100-x)}{100}$

$x=75.5$

$(100-x)=24.5$

The natural abundance of ${}^{35}Cl$ is $75.5$%