Please help me with this chemistry question?

interstellar space has an average temperature of about 10 K and an average density of hydrogen atoms of about one hydrogen atom per cubic meter. calculate the mean free path of hydrogen atoms in interstellar space.use d= 100 pm as the diameter of a hydrogen atom.

The equation for mean free path is:

λ=1/(sqrt(2)*pi*d^2*n)

or easier to see here:

http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic…

where λ is mean free path,
d is diameter of a hydrogen atom in m. (for you, 100pm = 10^-10 m)
and n is the # of atoms per cubic m (for you, 1 per m^3)

so just plugging these into the equation yields: 2.251 *10^19 meters.