# Homework Help: Number of molecular hits from air onto a surface

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1. Dec 15, 2015

### mr-serendip

1. The problem statement, all variables and given/known data
Compute the average number of molecular hits, per unit time, experienced by a square inch of surface exposed to air, under normal conditions. Assume air is a mixture composed of 80% N2, 20% O2, both of which are assumed to be ideal gases. You will have to perform angular averaging. You will also need to know the average speed, |v| of a molecule. If you know how to compute it, go ahead and do it. Otherwise, you may simply pretend that |v|= <v^2>1/2. Give answer in units of inverse seconds

2. Relevant equations
F=1/ 3* m<v^2> n S S=area of surface
P=1/ 3* m<v^2> n m=mass of particle n= concentration of particles
p=nKbT Nconctact = nvtS
3. The attempt at a solution
I've assume 293 degrees K and 101325 Pa
I've set both equations of pressure equal to each other and I get down to 2.67E25 particles / m^3
I then plug into the Ncontact equation the concentration I've found of particles, along with the area of the surface, and the average velocity. I'm not sure about this assumption but For my calculations for the molar mass I did .8*28+.2*32=28.8 g = .0288 kg.
For my answer I got 8.75E24 hits per unit second.
I do know that the answer is about 2.00E24 hits per unit second.
I'm not sure where I could have went wrong, I'm not sure if I am approaching this problem correctly or using the correct equations but I did get fairly close, which could be luck also. I would appreciate help, I've been stuck on this for a while now. Thanks

2. Dec 15, 2015