What is the concentration of conductions electrons in ntype

What is the concentration of conductions electrons in n-type sample of silicon, if one out of every million Si atoms is replaced by arsenic atoms.

Assume that the temperature is high enough that all the donor atoms are ionized.

The density of silicon is 2.33g/cm^3

Number of silicon atoms per cubic cm = density/molar mass*6.022*10^23=2.33/28.08*6.022*10^23=4.99*10^22

now concentration = free electron per arsenic atom*10^-6*number of silicon atoms

=1*10^-6*4.99*10^-6=4.99*10^16 elec/cm3