# Average atomic mass of chlorine is 35.5. It occurs in atmosphere in two isotopic forms 17cl35 and 17cl37 . The percentage abundance of these isotopes are respectively (1) 80% and20% (2)75% and25% (3) 70% and30% (4) 60% and 40%

2
by chandrasekhar

2014-10-01T19:44:53+05:30

### This Is a Certified Answer

Certified answers contain reliable, trustworthy information vouched for by a hand-picked team of experts. Brainly has millions of high quality answers, all of them carefully moderated by our most trusted community members, but certified answers are the finest of the finest.
When isotopes are present, atomic mass is taken as their weighted average.
Let percentage of 17c|35 chlorine = x%
percentage of 17c|37 chlorine = (100-x)% (as only 2 isotopes are there)

Atomic weight = [(35 × x) + ( 37× (100-x))] / 100
⇒35.5 = [35x + 3700 -37 x ] / 100
⇒3550 = -2x + 3700
⇒ 3550-3700 = -2x
⇒ 2x = 150
⇒ x = 75
100 - x = 25

So percentage of 17c|35 chlorine = 75%
percentage of 17c|37 chlorine = 25%