Chlorine has two isotopes having atomic masses of 35u and 37u each. They are present in the ratio 3:1 respectively. The average atomic mass of Chlorine is _______u.

The correct option is B i.e 35.5

Chlorine has two isotopes having atomic masses of 35u and 37u each, present in the ratio 3:1.The average atomic mass of Chlorine atom will be,
Average atomic mass = ($Massofisotope\times Relativeabundance$)
Relative abundance of isotopes of chlorine is in the ratio is - 3: 1
75 %: 25%
$\frac{75}{100}$: $\frac{25}{100}$
$\frac{3}{4}:\frac{1}{4}$

Hence, M= ((35$\times$ $\frac{3}{4}$)+(37$\times$$\frac{1}{4}$))

$\Rightarrow$ M= $\frac{105}{4}$+$\frac{37}{4}$

$\Rightarrow$ M= $\frac{142}{4}$

$\Rightarrow$ M= 35.5