What is the mass % of oxygen in lead (ii) nitrate (pb(no3)2) rounded to three significant figures?

The molar mass of Pb(NO₃)₂ can be calculated taking the atomic masses of the individual elements 
molar masses of the elements 
Pb - 207 g/mol
N - 14 g/mol
O - 16 g/mol
molar mass of the compound is the sum of the products of the molar masses by the number of atoms 
Pb(NO₃)₂ - 1 x 207 g/mol + 2 x 14 g/mol + 6 x 16 g/mol = 331 g/mol
the mass of 1 mol of Pb(NO₃)₂ has a mass of 331 g
1 mol of Pb(NO₃)₂ contains 6 mol of O 
mass of O in 1 mol of Pb(NO₃)₂ is - 96 g
mass percentage of O = mass of O / mass of Pb(NO₃)₂ x 100%
mass % = 96 g / 331 g x 100 % = 29.0 %
mass percentage of O is 29.0%

The problem here is that according to aufbau principle before 3d orbital 4s subshell comes.after filling 3p orbital with electrons you have 4 electrons left, you place two in 4s with opposite spin and the other two in 3d in any of its two orbitals of total five orbitals with the same spin.