Question

The pH of a compound is found to be 9. The compound can be
A.HCl
B.NaOH
C.$\text{N}{{\text{a}}_{\text{2}}}\text{C}{{\text{O}}_{\text{3}}}$
D.$\text{C}{{\text{H}}_{\text{3}}}\text{COOH}$

Answer 1

pH is the criteria which measure whether the given compound, solution, etc. falls under the category of acid or base. Acids are those which are capable of losing hydrogen ions and bases are those which are capable of losing $\text{O}{{\text{H}}^{-}}$ ion in the solution.

Complete answer:
pH scale ranging number from 0 to 14.
If $\text{pH}<\text{ 7}$ then it is said to be an acid
If $\text{pH}>\text{7}$ then it is said to be base
If $\text{pH}=7$ then it is neutral
Here the pH of the compound is found to be 9 then it belongs to the base and NaOH is the base.

So, ‘B’ is the correct option.

Additional information:
pH of solution is defined as the decimal logarithm of the reciprocal of hydrogen ion activity, ${{\text{a}}_{{{\text{H}}^{+}}}}$.
pH$=-\text{lo}{{\text{g}}_{10}}\left( {{\text{a}}_{{{\text{H}}^{+}}}} \right)=\text{lo}{{\text{g}}_{10}}\left( \dfrac{1}{{{\text{a}}_{{{\text{H}}^{+}}}}} \right)$
pH of solution changes with temperature, that is it depends on the temperature for example pH of water at 0 degree Celsius is $\text{7}.\text{47}$, pH of water at $25{}^\circ \text{C}$ is $\text{7}.00$ and at $100{}^\circ \text{C}$ is $\text{6}.\text{14}$. This is because as the temperature increases most of the water molecules converted to vapour and concentration of the water increases due to which pH changes.
This definition of pH was adopted because ion-selective electrodes, which are used to measure pH, respond to activity. Ideally, electrode potential, E, follows the Nernst equation, which, for the hydrogen ion can be written as
$\text{E}={{\text{E}}^{0}}+\dfrac{\text{RT}}{\text{F}}1\text{n}\left( {{\text{a}}_{{{\text{H}}^{+}}}} \right)={{\text{E}}^{0}}-\dfrac{2\cdot 303\text{RT}}{\text{F}}$ pH
Where E stands for electrode potential, ${{\text{E}}^{\text{0}}}$ is the standard electrode potential, R is
termed as gas constant, T is the temperature in kelvins, F is the Faraday constant.

Note: To solve this kind of problem we must know the pH scale and the values which fall under the acid, Base and neutral. The pH of water is 7 then it is neutral in nature. The pH scale tells us about the concentration of hydrogen ions present in the aqueous solution of the given compound.