Question

find the average atomic mass of chlorine atom if chlorine occurs in nature in two isotopic forms with masses 35u and 37u in the ratio 3:1 that is 75% and 25% respectively

Answer 1

there are two isotopes of chlorine which have different atomic masses such as 35 amu and 37 amu respectively.
in order to find the relative or average atomic mass of chlorine atom,
average atomic mass of chlorine equals to some of the atomic masses of both the isotopes divided by 2..
average atomic mass=35+37/2=36amu...
but basically the average atomic mass of CL is considered to be 35.5 a.m.u.
hope you like this...

Answer 2

[(35*75/100+37*25/100)=(105/4+37/4)=142/4=35.5