The isotope lithium-7 has a mass of 7.0160 atomic mass units,and the isotope lithium-6 has a mass of 6.0151 atomic mass units.Given the information that 92.58 percent of all lithium atoms found in nature are lithium-7 and 7.42 percent are lithium-6,calculate the atomic mass of lithium,Li (atomic number 3).
A)7.0160 amu
B)6.942 amu
C)6.495 amu
D)13.031 amu

Question

Quizplus · Accepted Answer

To find the atomic mass of lithium, we use the weighted average of the masses of its two isotopes, taking into account their relative abundances:

(0.9258)(7.0160 amu) + (0.0742)(6.0151 amu) = 6.939 amu

Rounding to three significant figures, the atomic mass of lithium is 6.942 amu, which is closest to choice B.

The Isotope Lithium-7 Has a Mass of 7