Charge carriers in semiconductors

Chemical analysis of a Si crystal reveals Ga at a level of 3•10−2 atomic percent. Assuming that the concentration of thermally excited charge carriers from the Si matrix is negligible, what is the density of free charge carriers/cm3 in this Si crystal?

Answer
Atomic weight of Si = 28
Density of Si = 2.33 g/cm3 = 5.01•1022 atoms/cm3
charge carriers / cm3 = 5.01•1022 × 3•10−4 = 1.503•1019

Solubility product (Ksp)

Lead chloride dissolves in water according to
PbCl2 (s) ↔ Pb2+ + 2 Cl-(aq)
The solubility in pure water has been measured to be 4.44 g⋅L-1.
Calculate the solubility product of lead chloride in pure water (in units of molarity)

Answer
44.4 g PbCl2 = 44.4 /278.2 = 0.016 moles
4.44 g/L = 0.016 M
[Pb2+] = 0.016 M
[Cl-] = 2 • 0.016 M = 0.032 M
K2=[Pb2+]•[Cl-]2 = 0.016 • (0.032)2 = 1.63 • 10-5 M3