To get the concentration of the second solution let us use the following formulae
C1V1=C2V2 where C1 is concentration of first solution and V1 is the volume of solution first solution. on the other hand C2 is the concentration of second solution and V2 is the volume of second solution.
therefore
0.8×2=(2+10)×C2
1.6 =12×C2
1.6/12=C2
C2 = 0.1333mg/mL
The formula unit of compound made up from Pb4+ and oxygen is PbO2.