Answer:- 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.