Question

There are two main isotopes of chlorine. 75% of chlorine atoms have a mass number of 35,while 25% have a mass number of 37 . Use this information to work out the relative atomic mass of chlorine.

Answer 1

HOW TO FIND RELATIVE ATOMIC MASS:

(mass of isotope × its relative abundance) + ( mass of isotope × it's relative abundance) ÷ sum of relative abundance:  

(35 × 75) +  (37 × 25) = 3550

sum of relative abundance= 75+25 =100

relative atomic mass of chlorine = 3550 ÷ 100 = 35.5

When you are doing it on a calculator you don't have to do it all separately, instead, use the fraction sign instead of ÷ ; Lay out the equation as a fraction.

Relative abundance means how much of it is found naturally. So for the chlorine isotope with mass number 35, its relative abundance is 75%.

Answer 2

Answer : The average or relative atomic mass of chlorine is 35.5 amu.

Explanation :

Average atomic mass of an element is defined as the sum of masses of each isotope each multiplied by their natural fractional abundance.

Formula used to calculate average atomic mass follows:

$\text{Average atomic mass }=\sum_{i=1}^n\text{(Atomic mass of an isotopes)}_i\times \text{(Fractional abundance})_i$

As we are given that,

Mass of isotope of chlorine-35 = 35 amu

Fractional abundance of isotope of chlorine-35 = 75 % = 0.75

Mass of isotope of chlorine-37 = 37 amu

Fractional abundance of isotope of chlorine-37 = 25 %  = 0.25

Now put all the given values in above formula, we get:

$\text{Average atomic mass of chlorine}=[(35\times 0.75)+(37\times 0.25)]$

$\text{Average atomic mass of chlorine}=35.5amu$

Therefore, the average or relative atomic mass of chlorine is 35.5 amu.