The molecular gas density at this pressure is 3.2
x 1013 molecules m-3
The gas density is directly proportional to the pressure , hence when the pressure of the gas is reduced by 3 decades from 10-6 Torr (as in the previous question) to 10-9 Torr, the gas density will also be lowered by a factor of 1000.
i.e. n = 3.2 x 1016 / 1000 = 3.2 x 1013 molecules m-3