Question

What is the \[\left[ OH- \right]\]concentration of \[NaOH\] solution with a \[pH\] of \[9.4\] ?

Answer 1

Hint: We know that 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 step-by-step answer:
We know that the degree of alkalinity or acidity of a solution is known as its pH. pH is the negative logarithm of the hydrogen ion concentration. We are given the concentration of sodium hydroxide \[NaOH\] solution. From the concentration of sodium hydroxide \[NaOH\] calculate the concentration of hydroxide ions. Then calculate the pH.

For beginnners, you know that an aqueous solution kept at room temperature has \[pH+pOH=14\]
So use this equation to find the \[~pOH\] of the solution; \[pOH=14-9.4=4.6\]
Now, the \[pOH\] of the solution is given by the concentration of hydroxide anions \[,~OH-\] : \[pOH=-log\left( \left[ OH- \right] \right)\]
To find the concentration of hydroxide anions starting from this equation, rewrite it as \[log\left( \left[ OH- \right] \right)=-pOH\]
$\Rightarrow 10log\left( \left[ OH- \right] \right)={{10}^{-pOH}}$ this will get you remember this equation \[\left[ OH- \right]={{10}^{-pOH}}\to ~\]
In your case, you have \[\left[ OH- \right]={{10}^{-4.6}}=2.5\times {{10}^{-5}}M\]

Note: Remember that if the pH of the solution is less than \[7\] then the solution is acidic in nature. If the pH of the solution is equal to \[7\] then the solution is neutral in nature. If the pH of the solution is more than \[7\] then the solution is basic in nature. Here, the pH is \[11\] thus, we can say that the solution is basic or alkaline in nature.