The balanced equation KHC8H4O4 + NaOH = NaKC8H4O4 + H2O shows how KHP and NaOH react. The chemical formula for KHP is KHC8H4O4, which stands for potassium hydrogen phthalate.
When KHP and NaOH mix, a positive hydrogen ion leaves the KHC8H4O4 and a negative hydrogen atom leaves the NaOH. The chemical formula for water is H2O, which is made up of these two atoms and the oxygen from the NaOH. Most of the time, a process called titration is used to bring about the reaction between the two solutions, so that the concentration and volume of each solution can be measured before and after the reaction. You can also call this reaction standard titration or just standardisation.