Question

Tellurium has eight isotopes: Te-120 (0.09%), Te-122 (2.46%), Te-123 (0.87%), Te-124 (4.61%), Te-125 (6.99%), Te-126 (18.71%), Te-128 (31.79%), and Te-130 (34.48%). What is its’ average atomic mass?

Answer 1

The average atomic mass of tellurium, calculated from its eight isotopes (Te-120 (0.09%), Te-122 (2.46%), Te-123 (0.87%), Te-124 (4.61%), Te-125 (6.99%), Te-126 (18.71%), Te-128 (31.79%), and Te-130 (34.48%)) is 127.723 amu.

The average atomic mass of Te can be calculated as follows:

$A = m_{Te-120}\%_{Te-120} + m_{Te-122}\%_{Te-122} + m_{Te-123}\%_{Te-123} + m_{Te-124}\%_{Te-124} + m_{Te-125}\%_{Te-125} + m_{Te-126}\%_{Te-126} + m_{Te-128}\%_{Te-128} + m_{Te-130}\%_{Te-130}$

Where:

m: is the mass

%: is the abundance percent

Knowing all the masses and abundance values, we have:

$A = 120*0.09\% + 122*2.46\% + 123*0.87\% + 124*4.61\% + 125*6.99\% + 126*18.71\% + 128*31.79\% + 130*34.48\%$

To find the average atomic mass we need to change all the percent values to decimal ones

$A = 120*9 \cdot 10^{-4} + 122*2.46 \cdot 10^{-2} + 123*8.7\cdot 10^{-3} + 124*4.61 \cdot 10^{-2} + 125*6.99\cdot 10^{-2} + 126*0.1871 + 128*0.3179 + 130*0.3448 = 127.723$

Therefore, the average atomic mass of tellurium is 127.723 amu.

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