an element X contains two naturally occurring isotopes 35/17 Z and 37/17 Z. If the average atomic mass of this element be 35.5 u. calculate the percentage of two isotopes.

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Average atomic mass = {mass number of 1st isotope * % of its existence /100} + { mass number of 2nd isotope * % of its existence/100}
 In the given question mass number of first isotope =35
In the given question mass numer of second isotope = 37
Let the % of first isotope  = x
let the % of second isotope = 100 - x
Average atomic mass = 35.5u
so 35.5 = {35 * x/100} + {37 * (100-x)/100}
35.5 = 35x/100 + 3700 - 37x/100
35.5 = 35x + 3700 -37x/100
35.5 *100 = 3700-2x
​3550 = 3700 -2x
3550 + 2x = 3700
2x = 3700 - 3550
2x = 150
x = 150/ 2
x = 75
First  % = x = 75
Second % = 100-x = 100 - 75 = 25
Ans - 35Z17 = 75
             37Z17= 25
  • * means multipled
  • / means divided
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