Find the average atomic mass for li if 7.5% of li atoms are 6li with a mass of 6.0151223 amu and 92.5% are 7li with a mass of 7.0160041 amu.

We solve this problem by taking the average atomic mass which is solved by the sum of the product of individual atomic mass and mass fraction, that is:

atomic mass = 6.0151223 amu * 0.075 + 7.0160041 amu * 0.925

atomic mass = 6.4898 amu