This resource consists of 2 worksheets: 25 calculation problems (and worked answers) involving aspects of acid base chemistry and some general pH calculation notes, which initially cover simple calculations. . The first half of each work sheet covers pH calculations in both acid and base conditions and calculation of H+ and OH- ion concentrations. The 2nd side of the worksheets focus on pH calculations involving partial neutralisation reactions where the student has to work out what species is in excess before calculating the pH. Some problems also involve the use of n=m/M equation. These problems would suit students doing IGCSE, year 11 chemistry or wanting to enter the National chemistry Olympiad competition. ps (June 2018) I have updated this resource and changed a problem that potentially is misleading.)