Question

What is the approximate \[[{H^ + }]\] in a solution with pH = 2?
A. \[1 \times {10^{ - 12}}M\]
B. 12M
C. \[1 \times {10^{ - 7}}M\]
D. \[1 \times {10^{ - 2}}M\]

Answer 1

Hint: pH is a scale used to measure acidity and alkalinity of a solution. The range goes from 0 to 14, where 7 is neutral, below 7 is acidic and above 7 is basic. It is the measure of concentration of hydrogen ions present in a substance.

Complete step-by-step answer:
Acids on dissociation produces hydrogen ions. Their concentration can be calculated if the pH of the solution is known. Thus, pH stands for potential of hydrogen. pH lower than 7 denotes that the solution is acidic. It becomes more acidic as we go from 7 to 0.
Due to an amphoteric nature of water, it reacts to form hydronium ions and hydroxide ions. This self-ionisation of water results in equal concentration of both the ions as \[1:1\] ratio. The experimental molarity is the same for both, \[1.0 \times {10^{ - 7}}M\] at room temperature. Their concentration product gives us a constant of water, \[{K_w}\]= \[1.0 \times {10^{ - 14}}M\]. This determines the range of the pH scale from 0 to 14 because \[p{K_w}\] = 14.
The solution we are provided with has a pH = 2, this shows that the solution is a strong acid. Its hydrogen ion concentration can be calculated by using the pH formula given below.
 \[pH = - \log ({H^ + })\]
 \[\therefore 2 = - \log ({H^ + })\]
Or \[\log ({H^ + })\]= -2
 \[\therefore [{H^ + }] = 1.0 \times {10^{ - 2}}M\] or 0.01M.

Hence, the correct option is (D).

Note: The letter p is written small in pH because it is a word from its meaning ‘power or strength of hydrogen’. Also note that \[p{K_w}\] = \[pH + pOH = 14\] and from this we can also calculate the concentration of hydroxide ions. It is a unitless quantity as it involves logarithm value only.