Question

What is the pH of a $25mL$ solution of $0.1N\,HCl$ ?

Answer 1

The concentration of hydrogen ions in a solution is measured by its pH, which is a measure of its acidity. Pure water splits into hydrogen and hydroxyl $(O{H^ - })$ ions in roughly equal amounts. The concentration of the hydronium ion in moles per litre is required to calculate the pH of an aqueous solution (molarity).

Complete answer:
The molarity of a material is equal to the amount of ${H^ + }$ from an incoming acid or $O{H^ - }$ from an incoming base that would be neutralised if the substance dissociates entirely.
In $HCl$ , there is only one ${H^ + }$ ; $0.1N\,HCl = 0.1M\,HCl$ .
If it was $0.1M\,{H_2}S{O_4}$ , then it would be $0.2\,N\,{H_2}S{O_4}$ , because $0.2M$ of $O{H^ - }$ is needed to neutralize $0.1M\,{H_2}S{O_4}$ .
As we know, $HCl$ is a strong acid, and it dissociates completely.
So, the dissociation reaction is given below:
$HCl(aq) \to {H^ + }(aq) + C{l^ - }(aq)$
Therefore, the molarity of $HCl$ , or [$HCl$ ], is pretty much the same as $[{H^ + }]$ .
$\Rightarrow [{H^ + }] = 0.1M$
It really doesn’t matter what the volume is. pH depends on the concentration, not the mols.
$pH = - \log [{H^ + }]$
$\,\,\,\,\,\,\,\,\, = - \log (0.1) = 1$
Hence, the pH of a $25mL$ solution of $0.1N\,HCl$ is 1.

Note:
The measurement of pH is now almost universally done by electronic means. The signal is converted to electric current via electrodes that are sensitive to hydrogen ion concentration (strictly speaking, hydrogen ion activity). In practically all steps of water and wastewater treatment, pH is critical.