Question

Gallium has two naturally occurring isotopes, Ga-69 with mass 68.9257 and Ga-71 withmass 70.9249. Calculate the percent abundances of these isotopes.

Answer 1

heya...!!
here's your answer!!

Average Atomic Mass of gallium =(68.9257+70.9249)/2
=139.8506/2
=69.9253

Now the game begins :)

Let x = % abundance of 1 st isotope

then (1-x) = % abundance of 2nd isotope

by using
AAM = (W1X1 * W2X2) / 100

69.70 = {70.925 * x + 68.926 * (1-x) } / 100
6970 = 70.925x + 68.926 - 68.926x
x = 61.56

61.56% is Ga-70.92. (100-61.56)
& 38.44% is Ga-68.93.

i hope that you'll understand !!