Indium (In) is made up of 95.7% 115In and 4.3% 113In. The atomic mass of 113In is 112.904 amu. The atomic mass of 115In is 114.904 amu. What is the average atomic mass of a sample of indium?

amu In = 114.818 amu

Explanation:

Indium (In):

∴ 95.7% 115In

∴ 4.3% 113In

∴ atomic mass 113In = 112.904 amu

∴ atomic mass 115In = 114.904 amu

average atomic mass (In):

⇒ amu In = (0.957) (114.904 amu) + (0.043) (112.902 amu)

⇒ amu In = 109.963 amu + 4.855 amu

⇒ amu In = 114.818 amu