The element antimony has an atomic weight of 122 and consists of two stable isotopes antimony-121 and antimony-123. the isotope antimony-121 has a mass of 121 amu and a percent natural abundance of 57.3 %. the isotope antimony-123 has a percent natural abundance of 42.8 %. what is the mass of antimony-123?

-  123 amu.

Solution:- The formula to calculate the average atomic mass of an atom is:

Average atomic mass = mass of first isotope(abundance of first isotope) + mass of second isotope(abundance of second isotope)

Note: The percent abundance is converted to decimals.

mass of Sb-121 is 121 amu and it's percent abundance is 57.3% and in decimal it is 0.573. Percent abundance of Sb-123 is 42.8% and in decimal it is 0.428. We are asked to calculate it's mass. The average atomic mass of Sb is given as 122.

Let's say the mass of Sb-123 is M and  plug in the values in the formula and do calculations:

122 = 121(0.573) + M(0.428)

122 = 69.333 + M(0.428)

On rearrangement:-

M(0.428) = 122 - 69.333

M(0.428) = 52.667

M = 123

So, the mass of Sb-123 is 123 amu.