What is the formula for calculating pH?
(A) $ pH = - \log [{H_3}{O^ + }] $
(B) $ dfrac{{pH}}{{\log }} = - \log [{H_3}{O^ + }] $
(C) $ pH = - \log [O{H^ - }] $
(D) $ pH = \dfrac{ - \log }{[{H_3}{O^ + }]} $
(E) $ pH = \log [{H_3}{O^ - }] $

Hint: One must know what pH represents. A pH stands for the power of hydrogen. pH denoting the potential of $ {\text{hydrogen}} $ is a scale used to specify the acidity or basicity of an aqueous solution. The acidic solution has lower pH and the basic solution has higher pH.

Complete answer:
The pH scale is logarithmic and inversely indicates the concentration of $ {\text{hydrogen}} $ ions in the solution. This is because the formula used to calculate pH approximates the negative of the base $ 10 $ logarithm of the activity of the molar concentration of $ {\text{hydrogen}} $ ions in the solution. In other words, pH is the negative of the base $ 10 $ logarithm of the activity of the $ {H^ + } $ ion. It is represented by $ - {\log _{10}}c $ where c is the $ {\text{hydrogen}} $ concentration in moles per litre.
It is the activity of the $ {H^ + } $ ion in the solution which accounts for the value of pH. It is given by negative logarithm of the molar hydronium ion concentration. In equation form, it is written as-
 $ pH = \log [{H_3}{O^ - }] $
Another way to represent pH is-
 $ pH = - \log [{H^ + }] $
In the above questions, only the first option is correct. If we look at the other options, they are the incorrect representation of the above formula of pH.
Therefore the correct answer is option A.

Note:
pH scale ranges from zero to fourteen, with seven being neutral. A pH of less than seven indicates acidity, whereas a pH of greater than seven indicates alkaline or basic. Acid donates protons or accepts a pair of valence electrons to form a bond whereas base accepts protons or donates a pair of electrons to form a bond.