Why is water ignored in the ionic equilibrium of HF?
Why are we not adding the contribution of water to the $\ce{H3O+}$ ions?
  



Write down the dissociation constant equation with and without $\ce{H2O}$.
What changes? 
Can you find a formulation of the dissociation constant where it seems natural not to include $\ce{H2O}$? 
Read up on "activity".



  
  Why are we assuming that the value $0.02x$ is dissociated from $\ce{HF}$ and not just $x$?
  


Write down and solve the equation with $0.02 x$ and with $x$ only. What is the difference?