Rms speed of Hydrogen atoms in space

[notsam]

Homework Statement

In outer space the density of matter is about one atom per cm^3, mainly hydrogen atoms , and the temperature is about 2.7 K. Calculate the rms speed of these hydrogen atoms, and the pressure (in atmospheres).

Homework Equations

PV = NkT

The Attempt at a Solution

So by using PV = NkT, I have all the variables except the volume. Or am I way off using this equation.

 

[ideasrule]

Have you encountered the equipartition theorem before? According to the theorem, hydrogen atoms have a kinetic energy of 3/2kT, because they have 3 degrees of freedom.

 

[notsam]

Ok...so 1/2(mv^2)=3/2kT. But what do I use for mass?