Question

If a 0.00001 molar solution of HCl is diluted thousand folds the pH of the resulting solution will be:
(A) 5
(B) 7
(C) 8
(D) 6.98

Answer 1

Hint: HCI is a strong acid. So the concentration of hydronium ion will be the same as that of HCI concentration. The formula to find the pH of the solution is as shown below. \[pH = - \log [{H^ + }]\]

Complete step by step solution:
We will find the pH of the given solution after diluting the solution.
- It is already given that the concentration of the solution is 0.00001 molar (${10^{ - 5}}M$). Now, it is given that the solution is diluted thousand folds. So, we can say that the concentration of the new diluted solution will be $\dfrac{{{{10}^{ - 5}}}}{{1000}} = {10^{ - 8}}$ M.
- We know that the pH of the solution is the measure of protons present in the given solution.
- We know that HCl dissolves to give protons and chloride ions. The dissociation reaction in aqueous medium can be given as below.
\[HC{l_{(aq)}} \to {H^ + }_{(aq)} + C{l^ - }_{(aq)}\]
- Scientist Sorenson defined that the negative logarithm of the concentration of hydrogen ions on the base of 10 present in the solution is the pH of the solution. So, we can write the formula of pH of the solution as
\[pH = - \log [{H^ + }]\]
Now, we know that the new concentration of HCl in the solution is ${10^{ - 8}}$ M. Now, HCl is a strong acid and it dissolves completely in aqueous medium. So, the concentration of protons in the solution should be ${10^{ - 8}}$ M. So, we can write the formula of pH as
\[pH = - \log [{10^{ - 8}}]\]
So,
\[pH = - ( - 8)\]
Thus, the pH of the solution will be 8.

So, the correct answer to the question is (C).

Note: Note that as pH is the negative logarithm of concentration of protons present in the solution, pOH is the negative logarithm of concentration of hydroxide ions present in the solution. We can also write hydronium ions $({H_3}{O^ + })$ in place of protons in the pH formula.