Question

The pH of 0.1 M NaOH is
(A) 13
(B) 12
(C) 11
(D) 10

Answer 1

Hint: pH denotes the ‘potential of Hydrogen’ or ‘power of Hydrogen’. The negative logarithm of the hydrogen ion activity is known as pH.
$pH=-\log [{{H}^{+}}]$

Complete step by step solution:
-Let us assume that the solution of 0.1M NaOH is a strong base and will produce 0.1mol/L of OH ions in the solutions.
$NaOH\to N{{a}^{+}}+O{{H}^{-}}$
Hence, $[O{{H}^{-}}]=[NaOH]=0.1M$
$pOH=-\log [1\times {{10}^{-1}}]$
$=-\log 1+-\log {{10}^{-1}}$
$=0+(-1)\log 10=0+1\times 1=0+1=1$
So, the $pOH$ of $0.1M$ solution of $NaOH$ is $1$.
-Now, we know that,
$pH + pOH = 14$
Therefore, we can now calculate the pH of 0.1M of NaOH from the calculated value of pOH and using the above equation,
$pH +1 = 14$
$\Rightarrow pH=14-1=13$
Therefore, the $pH$ of $0.1M$ $NaOH$ solution is $13$.

So, the correct answer is option A.

Note: You should not get confused between the pH and pOH of the solution.
-Both pH and pOH scales determine the measure of the strength of acids and bases between scale 1 to 14.
-pH is an abbreviation for the ‘potential of hydrogen’ while $pOH$ is an abbreviation for the ‘potential of hydroxide’.
-The pH for any aqueous solution is calculated from $[{{H}^{+}}]$ using the formula, $pH=-\log [{{H}^{+}}]$
The lowercase p indicates $'-{{\log }_{10}}'$ .
The pOH is defined as the negative logarithm of the hydroxide ion and is calculated using the following formula,
$pOH=-\log [O{{H}^{-}}]$
-Based on the equilibrium concentration of ${{H}^{+}}$and $O{{H}^{-}}$ions in water, the following relationship is true for any aqueous solution at $25{}^\circ C$.
$pH + pOH = 14$
-Anything with a pH between 1 to 6 is acidic while anything with the pOH between 1 to 6 is basic.
-Anything with a pH between 8 to 14 is basic while anything with the pOH between 8 to 14 is acidic.
-Anything with a pH and pOH 7 is neutral.
-If an acid has a pH between 1 to 3, it is a strong acid, while a base has a pOH between 1 to 3 is a strong base.
-If an acid has a pH between 4 to 6, it is a weak acid, while a base has a pOH between 4 to 6 is a weak base.
-If a base has a pH between 8 to 11, it is a weak base, while an acid has a pOH between 8 to 11 is a weak acid.
-If a base has a pH between 12 to 14, it is a strong base, while an acid has a pOH between 12 to 14 is a strong acid.