Question

Lithium-6 has a mass of 6.0151 amu and lithium-7 has a mass of 7.0160 amu. The relative abundance of Li-6 is 7.42% and the relative abundance of Li-7 is 92.58%. Based on this data alone, calculate the average atomic mass for lithium to the correct number of significant digits.

Answer 1

Average atomic mass is the weighted average atomic masses with regard to the relative abundance of the isotopes 
average atomic mass of Li = relative abundance of Li-6 x mass of Li-6 + relative abundance of Li-7 x mass of Li-7
average atomic mass of Li = (7.42% x 6.0151 a.m.u) + (92.58% x 7.0160 a.m.u)
                                           = 0.446 + 6.495
                                           = 6.941 amu
average atomic mass of Li is 6.941 amu

Answer 2

Answer: The average atomic mass of Lithium is 6.94 amu.

Explanation:

Mass of isotope Li-6 = 6.0151 amu

% abundance of isotope  Li-6 = 7.42%= $\frac{7.42}{100}=0.0742$

Mass of isotope Li-7=  7.0160

% abundance of isotope Li-7 = 92.58% = $\frac{92.58}{100}=0.9258$

Formula used for average atomic mass of an element :

$\text{ Average atomic mass of an element}=\sum(\text{atomic mass of an isotopes}\times {{\text { fractional abundance}})$

$A=\sum[(6.0151\times 0.0742)+(7.0160\times 0.9258)]$

$A=6.94$

Therefore, the average atomic mass of Lithium is 6.94 amu.