Question

Chlorine exists in two isotopic forms Cl-37 and Cl-35, but its atomic mass is 35.5. This indicates the ratio of Cl-37 and Cl-35 is approximately
(a) 1 : 2
(b) 1 : 1
(c) 1 : 3
(d) 3 : 1

Answer 1

Answer:(c) 1 : 3 is the correct answer.

Explanation:

Isotopes are the elements having same atomic number but different atomic mass number.

The two isotopes of chlorine are

Cl37 and Cl35.

Thus they have different atomic mass number.

The average atomic masses are calculated by multiplying atomic masses with their respective relative abundance.

The relative abundance of Cl37 is roughly 24%.

The relative abundance of Cl35 is roughly 76%.

Thus average atomic mass= (37*24/100) +(35*76/100)

=35.5

Also

If the ratio is 1:3

Average atomic mass will be caluclated as

(37*1) +(35*3) / 1+3

=37+105/4

=142/4

=35.5 which is as given.

Hence (c) 1 : 3 is the correct answer.

Answer 2

Answer: The correct answer is Option c.

Explanation:

To calculate the ratio of abundances of the given isotopes, we use the equation given by average atomic mass, which is:

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

Let the fractional abundance for Cl-35 isotope be 'x'.

So, fractional abundance for Cl-37 isotope will be (1 - x)

• For Cl-35 isotope:

Mass of Cl-35 isotope = 35 u

Fractional abundance of Cl-35 isotope = x

• For Cl-37 isotope:

Mass of Cl-37 isotope = 37 u

Fractional abundance of Cl-37 isotope = 1 - x

Average atomic mass of chlorine = 35.5

Putting values in above equation, we get:

$35.5=[(35\times x)+(37\times (1-x))]\\\\x=0.75$

Fractional abundance for Cl-37 isotope = 1 - 0.75 = 0.25

Percentage abundance for Cl-35 isotope = 75 %

Percentage abundance for Cl-37 isotope = 25 %

The ratio of Cl-37 and Cl-35 isotopes = $\frac{0.25}{0.75}=\frac{1}{3}$

Thus, the correct answer is option c.