The molecular formula of is ‎potassium acid phthalate
C8H5KO4
Molar mass of potassium acid phthalate is 204 g/mol
So number of moles of potassium acid phthalate , n = mass/molar
mass
                                                                          Â
= 4.000 g / 204 (g/mol)
                                                                         Â
= 0.0196 mol
Which is equivalent to 48.37 ml of NaOH
So Molarity of NaOH ,M = number of moles / volume in L
                                  Â
= 0.0196 mol / ( 48.37/1000) L
                                  Â
= 0.405 M
Therefore the molarity of NaOH is 0.405 M