# Homework Help: Finding what fraction of molecules have speeds over a certain value

1. May 13, 2013

### Erubus

1. The problem statement, all variables and given/known data
A sample of oxygen gas with molar mass 32 g/mol contains 5.0 moles and is at a temperature of 500 K. How many moles of the gas have speeds between 375 m/s and 380 m/s?

2. Relevant equations
Maxwell Boltzmann Distribution:
f(v) = 4$\pi$[$\frac{M}{2pi RT}$]^(3/2) * v^2 * e^[(-Mv^2)/(2RT)]

3. The attempt at a solution
I know that this function gives the probablility of having the molecules at some speed v, but I'm not sure how to translate this over a range or how to calculate the number of moles over this range.

2. May 14, 2013

### SteamKing

Staff Emeritus
Do you know how to use the M-B distribution to find out how many molecules have a speed greater than 380 m/s?

Do you know how to use the M-B distribution to find out how many molecules have a speed greater than 375 m/s?

3. May 14, 2013

### Erubus

I figured it out, thanks!