The complex formation between a weak polyelectrolyte chain and an oppositely charged nanoparticle is investigated using Monte Carlo simulations. Global structural parameters such as the polyelectrolyte length, nanoparticle size, solution pH, and ionic concentration as well as local features, such as the nanoparticle surface charge density and polyelectrolyte intrinsic stiffness influences, are systematically investigated. Phase states of the polyelectrolyte/nanoparticle complexes are presented, and to bridge the gap with experiments, titration curves are calculated. It is shown that the presence of one oppositely charged nanoparticle significantly modifies the acid/base properties of the weak polyelectrolyte as well as the charge distribution along the polymer backbone and that the solution pH and ionic concentration largely control the polyelectrolyte conformation at the nanoparticle surface. Chain stiffness promotes the polyelectrolyte expansion as well as ionization but penalizes the polyelectrolyte adsorption at the nanoparticle surface, hence affecting its acid/base behavior.