An aqueous solution of KOH has a hydroxide (OH-) concentration of 0.02 mole/L. The hydroxide ion is the OH- portion of KOH. When KOH (potassium hydroxide) is dissolved in water is dissociates into its respective ions, potassium and hydroxide. The ion dissociation is shown in the chemical equation below.
KOH ---> K+ + OH-
We can look at the above equation and learn that one mole of potassium hydroxide produces one mole of potassium cations and one mole of hydroxide anions. So if we have a solution that is 0.02 mol/L in hydroxide ions, we know that it was originally 0.02 mol/L concentration in potassium hydroxide since they are related to one another on a one to one molar ratio. The concentration of potassium ions would also be 0.02 mol/L as well.