Question

Which of the following pH values at $25^\circ C$ corresponds to the strongest acid?
A. 2
B. 4
C. 6
D. 8

Answer 1

pH is the value that expresses the acidity or alkalinity of a solution on a logarithmic scale. If the pH is 7, this means that the solution is neutral. If $pH < 7$ the solution is acidic whereas if the $pH > 7$ the solution is alkaline or basic.

Complete step by step answer:
The higher the number of ${H^ + }$ ions present, the greater is the acidic character of that compound. Mathematically, it is equivalent to the negative logarithmic value of the concentration of the ${H^ + }$ ions in a compound. It can be represented as:
$pH = - \log [{H^ + }]$
pH is measured on a scale of 1 to 14, where 1 means the most acidic while 14 means most basic. Chemically neutral substances lie at the center of this scale, i.e. at 7. This because the acidic character of a neutral substance is balanced by its basic character. To put it in simpler terms, let us understand another small concept: the basic character of a substance is determined by the concentration of $O{H^ - }$ ions in the compound and is put in perspective by a measure known as the pOH scale.
So, in neutral substances, the concentration of $O{H^ - }$ and ${H^ + }$ is equivalent and they cancel each other out to give the neutral characteristic.
Therefore, according to the pH scale, the lower the value of the pH, the higher will be the acidity of the acid and vice versa. So, considering this fact among the options the pH value for the strong acid is 2.

So, the correct option is A.

Note: We know that water is a neutral substance and hence its pH is 7. And as per the explanation above, the number of ${H^ + }$ ions is equal to the number of $O{H^ - }$ ions and hence they cancel out their acidic and basic properties to give water its neutral nature.