Question

How is the concentration of hydroxide ions $\left( O{{H}^{-}} \right)$ affected when excess base is dissolved in a solution of sodium hydroxide?

Answer 1

To solve this identify the kind of ions furnished by sodium hydroxide in a solution. Check whether it’s an Arrhenius acid or base. If it is an Arrhenius base then the concentration of hydroxide ions will increase and if it’s an Arrhenius acid then the concentration of hydroxide ions will decrease.

Complete step by step answer:
Before answering this question, let us understand what a base is. There are several theories which we can use to describe a base but here we will use the Arrhenius theory.
According to the Arrhenius theory, hydroxide ion donors in aqueous solution are bases. An Arrhenius base will increase the concentration of hydroxide ions in the solution.
Now let’s try to answer the given question.
We know that sodium hydroxide is NaOH. In aqueous solution it will furnish $O{{H}^{-}}$ ions.
Now, if we dissolve excess base (i.e. compounds furnishing $O{{H}^{-}}$ ions in solution) in a solution of sodium hydroxide which is a base itself, then of course the concentration of hydroxide ions will increase. This statement will hold true only if the base is soluble in the solution. If it cannot dissolve then it will nor furnish any ions.
However, it is mentioned in the question that the base is dissolved in the solution.
Therefore, we can understand from the above discussion that the concentration of hydroxide ions will increase in the solution and this is the required answer.

Note: There are various theories that give us the idea about base (and acid) depending upon different factors. For example, according to Lewis theory of acidity and basicity, the species which lose electrons to get a stable configuration are Lewis bases. According to the solvent system theory, a solute that furnishes the anion characteristic of the solvent either by direct dissociation or by reaction with solvent is a base.