I believe we are given the following data:
82p, 122n, 1.4%
82p, 124n, 24.1%
82p, 125n, 22.1%
82p, 126n, 52.4%
Where p stands for proton and n for neutrons. We know that
the mass number (atomic mass) is equivalent to the sum of p and n, hence the atomic
mass of each isotope is:
isotope 1: 82 + 122 = 204 amu
isotope 2: 82 + 124 = 206 amu
isotope 3: 82 + 125 = 207 amu
isotope 4: 82 + 126 = 208 amu
The atomic mass of Lead is the sum of the weighted average
atomic mass of the isotopes, that is:
atomic mass Lead = 204 * 0.014 + 206 * 0.241 + 207 * 0.221 +
208 * 0.524
atomic mass Lead = 207.241 amu = 207.241 g/mol
