Answer:
[tex]\boxed{\text{121.8 u; Sb}}[/tex]
Explanation:
The atomic mass is the weighted average of the atomic masses of each isotope.
In a weighted average, we multiply each value by a number representing its relative importance.
In this problem, the percent abundance represents the relative importance of each isotope.
Data:
X1 = 57.4 %; mass = 120.9038 u
X2 = 122.9042 u
Calculations:
(a) Calculate X2 abundance
X2 = 100 - 57.4 = 42.6 %
(b) Calculate the atomic mass of X
[tex]\begin{array}{cccc}\\\textbf{Isotope} & \textbf{Mass/u} & \textbf{Abundance} & \textbf{Contribution/u}\\\text{X1} & 120.9038 & 0.574 & 69.4\\\text{X2} & 122.9042 & 0.426 & 52.4\\& &\text{TOTAL =}& \mathbf{121.8}\\\end{array}\\\text{The average atomic mass of the sample is }\boxed{\textbf{121.8 u}}[/tex]
The element is antimony. Sb has two stable isotopes with the given abundances and an average atomic mass of 121.76 u.
Note: The answer can have only one decimal point because the isotopic abundances have only three significant figures.
