Question

The concentration of hydroxide ions $\left( {O{H^ - }} \right)$is affected when an excess base is dissolved in a solution of sodium hydroxide. The concentration of base in the new solution will
A.Increase
B.Decrease
C.Remain same
D.None of these

Answer 1

To answer this question, you must recall the Arrhenius concept of classification of substances into acids or bases. According to this theory, the substance that produces a hydrogen ion in solution is known as an acid and that which produces a hydroxide ion is a base.

Complete answer:
Arrhenius acid is a compound which when dissolved into an aqueous solution, releases hydrogen ions in the solution. These ${H^ + }$ ions combine with water molecules and form hydronium ions$\left( {{H_3}{O^ + }} \right)$. Similarly an Arrhenius base is a compound which when dissolved into an aqueous solution, releases hydroxide $\left( {O{H^ - }} \right)$ ions in the solution.
The dissociation of $NaOH$ in aqueous solution can be represented as
${\text{NaOH}}\left( {aq} \right) \to {\text{N}}{{\text{a}}^ + }{\text{(aq)}} + {\text{O}}{{\text{H}}^ - }({\text{aq)}}$
So we can say from the above equation, that addition of a base to the solution results to an increase in the concentration of the hydroxide ions in the solution. When we add excess amounts of base into the solution, the concentration of hydroxide ions will increase. However this is possible only if the base is soluble in water. If the base does not dissolve in water, then the concentration of hydroxide ions in the solution remains constant.

Thus, the correct option is A.
Note:
The concentration of hydroxide ions in a solution is inversely proportional to that of hydroxide ions. If the concentration of hydrogen ions in a solution increases, the solution becomes more acidic and if the concentration of hydroxide ions in a solution increases, the solution becomes more alkaline.