Isopiestic determination of osmotic coefficients in the ionic strength range Im = (0.9670–2.2160) mol⋅kg−1 and activity coefficients determined by electromotive force measurements in the range Im = (0.0897–1.0054) mol⋅kg−1 of the {yKCl + (1 – y) K2HPO4}(aq) system at T = 298.15 K

•The osmotic coefficients of the system {yKCl+(1–y)K2HPO4}(aq) atT = 298.15 K determined by the isopiestic method.•Electromotive force measurement in the cell.•K–ISE∣KCl(mKCl), K2HPO4(mK2HPO4)∣Ag∣AgCl for KCl activity coefficient estimate.•Extended Pitzer’s, Scatchard’s and Clegg–Pitzer–Brimblecombe’s models in treatment of activity and osmotic coefficient data.•Model’s parameter value optimization by treatment of all available thermodynamic properties.