# Number density of earth

I was wondering what is the best way to calculate the number density of earth? Should we assume that the earth is mostly Iron and calculate the N from there or should we assume the average particle mass is the mass of a proton?

Also if a small black hole is created on earth and makes one passage through earth how can we calculate the mass it sweeps up? I calculated the mean free path of this black hole created from two protons colliding and I get a mean free path that is extremely large >>>>>> the diameter of earth. Does this mean the black hole will not interact with anything and go right through earth? I am not quite sure about this because don't black holes oscillate?