How can we synthesize para-nitrophenol
This can be done by first reating the phenol with $\ce{HNO2}$ Which gives us this  

  

Then reacting with dil $\ce{HNO3}$ we get 90% yield of para nitro phenol  

This happens because $\ce{N=O}$ has a very strong $- I$  effect thus it concentrates the electrons more towards the para position so on reacting with dil $\ce{HNO3}$ we get max yield of para  

P.S - Another way of getting only para is using a protecting agent but  i cant recall the reaction now. I will post it later