# Homework Help: Pressure due to molecular velocity problem

1. Feb 19, 2006

### pmastchief

Hey guys,

I am having a little problem with a problem I am doing in Physics (thermo chapter).

Well, here is the problem:

"The mass of the H2 molecule is 3.3e24 g. If 10e23 H2 molecules per second strike 2.0 cm^2 of wall at an angle of 55° with the normal when moving with a speed of 1.7 10e5 cm/s, what pressure in Pascals do they exert on the wall?"

I have attempted this problem numerous times for atleast 2 hours and 45 minutes and have not been able to get the correct solution.

I keep on getting 2.51e5 Pa but it is not correct.

If anyone could shed some light on this problem, I would greatly appreciate it!! I am more interested as to how you arrive to the solution rather that just an answer.

thanks,
joe

2. Feb 19, 2006

### Staff: Mentor

Show how you arrived at your answer. (Hint: Consider the change in momentum of the molecules as they bounce off the wall.)

3. Feb 19, 2006

### pmastchief

The value of the velocity that is used during momentum calculation is the velocity in the x direction which would be :

Vx = 170 m/s * cos (55 degrees)
I converted the velocity from centimeters to meters
(velocity in centimeters = 170000 cm/s)

and the pressure exerted is:
P = [ n*m*(Na) / L^3 ] * Vx^2

where
n is the number of moles
m is the mass of the molecules
Na is Avogadro's number
Vx is the calculation above

However, I just realized that the L is the length and I was using the area that is given to me in the problem. But how can I find the pressure if I do not have the length of one side?

thanks

4. Feb 19, 2006

### phucnv87

$$N\Delta p=F\Delta T=F$$

where $$p$$ is the momentum

The pressure is $$P=\frac{F}{1m^2}=F$$

You just need to calculate the $$\Delta p$$

Last edited: Feb 20, 2006
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