Question

multiple choice question
the element chlorine is made up of two isotopes, 0.25 of which have an atomic mass of 37.0 amu and 0.75 of which have an atomic mass of 35.0 amu. what is the average atomic mass of chlorine?
36.0 amu
35.5 amu
36.6 amu
37.0 amu

Explanation:

Step1: Recall average - atomic - mass formula

The formula for the average atomic mass of an element with two isotopes is $A_{avg}=x_1A_1 + x_2A_2$, where $x_1$ and $x_2$ are the fractional abundances of the isotopes and $A_1$ and $A_2$ are their respective atomic masses. Given that $x_1 = 0.25$ and $A_1=37.0$ amu, and $x_2=1 - 0.25=0.75$. The other common isotope of chlorine has an atomic mass of approximately 35.0 amu.

Step2: Calculate average atomic mass

$A_{avg}=0.25\times37.0+0.75\times35.0$
$A_{avg}=9.25 + 26.25$
$A_{avg}=35.5$ amu

Answer:

B. 35.5 amu