You are messing up several things.
1st - molar mass is 315.4 gram per mol, not 315.4 mol.
2nd - you didn't tell how many moles is 3.9g, but according to your concentration calculations you have found it to be 0.02276 mole - which is wrong.
3rd - barium hydroxide dissociation ends up with OH- ions, not H+ - so you have to use Kw in your calculations.