You can see the periodic table one by one.
Pb(NO3)2 = PbN2O6
So there are
1Pb
2N
6O
and so you find the molecular mass from the periodic table and add all the numbers
Molacular mass of
Pb = 207
N = 14
O = 16
multiply it with the number of atoms
1Pb = 207 × 1 = 207
2N = 14 × 2 = 28
6O = 16 × 6 = 96
Molecular mass total = 207 + 28 + 96 = 331
So the molecular mass is 331.
