Question

When one drop of a concentrated HCl is added to 1 L of pure water at 25$^{0}C$ , the pH drops suddenly from 7 to 4. When the second drop of the same acid is added, the pH of the solution further drops to about
a.) 0
b.) 1.0
c.) 2.0
d.) 3.7

Answer 1

Hint: Before solving this question, first we have to understand what is pH. pH stands for power of hydrogen and it is written as small “p” followed by a capital “H”.

Complete step by step answer:
pH is the scale which is used to specify the acidity and basicity of a solution. The acidic solutions generally have lower pH value while the basic solutions have higher pH value of the pH scale.
Given,
Initial pH =7
\[[{{H}^{+}}]={{10}^{-4}}\]
Change in the concentration $[{{H}^{+}}]={{10}^{-4}}-{{10}^{-7}}={{10}^{-4}}(1-{{10}^{-3}})={{10}^{-4}}$
i.e. the change in the concentration of the hydrogen ion in one drop = ${{10}^{-4}}$
change in concentration after 2 drops = $2\times {{10}^{-4}}$
hence, the pH of the solution will be:
\[\begin{align}
  & pH = -\log [{{H}^{+}}]=-\log (2\times {{10}^{-4}}) \\
 & = 4 - \log 2 \\
 & = 4 - 0.3 \\
 & = 3.7 \\
\end{align}\]
The correct option is option “D” .

Note: The pH scale generally ranges from 0 to 14. It helps us to determine how acidic or basic the solution is. If the pH of the solution is less than 7, then it is acidic and if the pH of the solution is more than 7, then it is the basic solution. Similarly, if the pH of the solution of equal to 7, then the solution is neutral. The formula for calculation if pH is =
\[pH = -\log [{{H}^{+}}]\]
Similarly, for a base we can calculate the concentration of OH ions using molarity and the calculate pH using the value of pOH.