The number of molecules crossing a unit area per unit time in one direction is given by
where n is the number of molecules per unit volume and v is the average molecular velocity.
As the average molecular velocity is approximately equal to the speed of sound in a perfect gas, estimate the number of molecules crossing a circular hole 10-3 in. in diameter.
Assume that the gas is at standard conditions. At standard conditions, there are 4 × 1020 molecules per in.3.