We describe the pH dependence of the electron mobility as a function of various structural and electrolyte factors for unfunctionalised AlGaN/GaN-based chemical sensors. The charge distribution across the sensor-electrolyte system is calculated self-consistently and the dependence of the Hall mobility of the two-dimensional electron gas on different scattering mechanisms is studied. Our calculations show that the pH of the electrolyte has a significant effect on the mobility and consequently the conductivity of the 2DEG channel electrons. A wide range of structures is examined to describe the correlation between the sensitivity and channel electron density. It is shown that the change of mobility with pH is a strong function of the structure of the sensor (AlGaN, GaN-cap and oxide-layer thicknesses), while it is almost independent of the ionic strength of the electrolyte. The observed dependence of the Hall-mobility to the thickness of AlGaN-barrier, GaN-cap and oxide layer can explain the discrepancy in the behaviour of the Hall-mobility, as reported by different groups.