Isotope Atomic Mass (amu) Percent Abundance Silicon-28 27.98 92.21%

Silicon-29 28.98 4.70%

Silicon-30 29.97 3.09%
Look at the information given and predict whether the average atomic mass would be closer to Si-28, Si-29, or Si-30. Give reasons for your answer.

The average atomic mass is closer to Si- 28 because this isotope is present in more percentage in the sample.

Explanation:

Given data:

Atomic mass of silicon= ?

Percent abundance of Si-28 = 92.21%

Atomic mass of Si-28 = 27.98 amu

Percent abundance of Si-29 = 4.70%

Atomic mass of  Si-29 = 28.98 amu

Percent abundance of Si-30 = 3.09%

Atomic mass of  Si-30 = 29.97 amu

Solution:

Average atomic mass = (abundance of 1st isotope × its atomic mass) +(abundance of 2nd isotope × its atomic mass)+(abundance of 2nd isotope × its atomic mass)  / 100

Average atomic mass = (92.21×27.98)+(4.70×28.98)+(3.09×29.97) /100

Average atomic mass =  2580.04 +136.21+92.61 / 100

Average atomic mass = 2808.86 / 100

Average atomic mass  = 28.08amu.

The average atomic mass is closer to Si- 28 because this isotope is present in more percentage in the sample.