Question

the element lithium is composed of 2 isotopes; about 92.41% of all lithium atoms are $^7$li with a mass of 7.0160 amu, and the remaining 7.59% of lithium atoms are $^6$li with a mass of 6.0152 amu. the average mass of lithium is ________. select the correct answer below: 6.484 amu 0.4825 amu 6.94 amu none of the above

Explanation:

Step1: Convert percentages to decimals

$92.41\% = 0.9241$, $7.59\%=0.0759$

Step2: Calculate weighted - average mass

$m = 0.9241\times7.0160+0.0759\times6.0152$
$m = 0.9241\times7.0160 = 6.4834856$
$m = 0.0759\times6.0152 = 0.45655368$
$m=6.4834856 + 0.45655368=6.94003928\approx6.94$ amu

Answer:

6.94 amu