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What are the natural abundances of the other two isotopes? 25Mg and 26Mg. In %.

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There are 3 stable isotopes of magnesium. Their masses are 23.9850, 24.9858 and 25.9826 amu. If the average atomic mass of magnesium is 24.3050 amu and the natural abundance of the lightest isotope is 78.99%.

Oct 5th, 2014

Given Magnesium 24 is in abundance of 78.99 %.

Therefore amount remaining = 100-78.99%=21.01%

Now this 21% is equal to (Magnesium 25 + Magnesium 26)

take Mg-25 = X

and Mg-26=21.01 - X

Given average atomic mass of magnesium = 24.3050 amu

24.3050 = (0.7899) x (23.9850) + (X) x ( 24.9858) + (0.2101-x) x 25.9826

24.3050 = 18.9457 + 24.9858 X + [ ( 25.9826 x 0.2101) - (25.9826 x X)]

24.3050 = 18.9457 + 24.9858X + 5.4589 - 25.9826 X

24.3050=18.9457 +5.4589 +24.9858X-25.9826X

24.3050 = 24.4046-0.9968X

24.3050-24.4046 = -0.09968X

-0.0996=-0.09968X

X=-0.0996/-0.08868=0.09991


therefore Mg-25= X = 0.09991 Multiply by 100 = 9.991%

 and Mg-26 =21.01-X=21.01-9.991 = 11.019%



Oct 5th, 2014

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