For a non-volatile solute in a volatile solvent: P_solution = x_solvent * P_solvent_standard. RLVP: (P_standard - P_solution)/P_standard = delta_P/P_standard = x_solute = n_solute/(n_solute + n_solvent). For dilute solutions: delta_P/P_standard ≈ n_solute/n_solvent = (w_solute/M_solute)/(w_solvent/M_solvent). This is used to find molar mass of non-volatile solutes. For electrolytes: delta_P/P_standard = i * x_solute. Important: RLVP applies only when the solute is non-volatile. If both components are volatile (like benzene + toluene), use Raoult's law for both — RLVP formula does not apply directly.