How to find the percentage of an isotope?

Question

I have two isotopes one Cl-37 and Cl-35 atomic unt is 35.45

and one part in the equation there (1-x) how do i get that ?

Answer 1

Since the weight for chlorine is 35.45 you can easily see that it is mostly the Cl-35 isotope.

To calculate:

Let x = the fractonal amount of Cl-35

so, 1 - x = fractional amount of Cl=37

Making the equation:

35x + 37(1-x) = 35.45

35x + 37 - 37x = 35.45

37 - 2x = 35.45

-2x = -1.55

x = 0.775

Or 77.5% Cl-35 and 0.225% Cl-37

Check it:
(0.775 x 35) + (0.225 x 37) = 35.45

Answer 2

Let x equal the percentage of Cl-37
Let y equal the percentage of Cl-35

The percentages must add up to 1, so -
x + y = 1 so, y = 1-x

When you multiply the weight of the isotope by its percentage and add together, you get the average atomic mass -
37x + 35y = 35.45

Now you have a simultaneous equation

37x +35(1-x) = 35.45
37x + 35 - 35x = 35.45
2x + 35 = 35.45
2x = 0.45
x = 0.225 (percentage of Cl-37)

so 0.225 + y = 1
y = 0.775 (percentage of Cl-35)

Hope this helps

Answer 3

here is my webpage on atomic mass calculations and % abundance of isotopes

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