Question

li-6 has an atomic mass of 6.015 amu and a relative abundance of 7.5% while li-7 has an atomic mass of 7.016 amu and a relative abundance of 92.5%. the average atomic mass for li is

a 6.94 amu

b 6.11 amu

c 6.83 amu

d 6.25 amu

Explanation:

Step1: Convert percentages to decimals

$7.5\% = 0.075$, $92.5\%=0.925$

Step2: Use the formula for average atomic mass

Average atomic mass = (Mass of Li - 6×Abundance of Li - 6)+(Mass of Li - 7×Abundance of Li - 7)
$=(6.015\times0.075)+(7.016\times0.925)$
$= 6.015\times0.075=0.451125$
$=7.016\times0.925 = 6.4898$
$0.451125 + 6.4898=6.940925\approx6.94$ amu

Answer:

A. 6.94 amu