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