Question

Calculate the average atomic mass of unknown element X. You determine that isotope 127X makes up 80%, isotope 126X makes up 17%, and isotope 128X makes up 3%.

A. 127.10 amu
B. 126.86 amu
C. 127.54 amu
D. 126.20 amu

Answer 1

Option B. 126.68 amu

Step-by-step explanation:

The following data were obtained from the question:

Isotope A (¹²⁷X):

Mass of A = 127

Abundance (A%) = 80%

Isotope B (¹²⁶X):

Mass of B = 126

Abundance (B%) = 17%

Isotope C (¹²⁸X):

Mass of C = 128

Abundance (C%) = 3%

Average atomic mass of element X =.?

Th4 average atomic mass of X can be obtained as follow:

Average atomic mass = [(Mass of A × A%)/100] + [(Mass of B × B%)/100] + [(Mass of C × C%)/100]

= [(127 × 80)/100] + [(126 × 17)/100] + [(128 × 3)/100]

= 101.6 + 21.42 + 3.84

= 126.68 amu

Therefore, the average atomic mass of X is 126.68 amu