Which of the following has the highest value of pH?

The correct option is B

$\mathrm{NaOH}$

The explanation for the correct option (B):

• pH of the solution is used to determine the acidic or basic strength of a solution.
• When the pH of the solution is less than 7, the solution is said to be acidic.
• When the pH of the solution is above 7, the solution is basic and when it is equal to 7, the solution is neutral.
• Sodium hydroxide ($\mathrm{NaOH}$) is a strong base as it completely ionizes in an aqueous solution to produce Hydroxide (${\mathrm{OH}}^{-}$) ions.
• $\mathrm{NaOH}\left(\mathrm{aq}\right)\to {\mathrm{Na}}^{+}\left(\mathrm{aq}\right)+{\mathrm{OH}}^{-}\left(\mathrm{aq}\right)$
• Since $\mathrm{NaOH}$ is strongly basic, it has the highest pH (above 7) among the given options.

The explanation for the incorrect options:

(A)

• Hydrochloric acid ($\mathrm{HCl}$) is a strong acid as it completely ionizes in an aqueous solution to produce Hydrogen (${\mathrm{H}}^{+}$) ions.
• $\mathrm{HCl}\left(\mathrm{aq}\right)\to {\mathrm{H}}^{+}\left(\mathrm{aq}\right)+{\mathrm{Cl}}^{-}\left(\mathrm{aq}\right)$
• Since $\mathrm{HCl}$ is a strong acid, the value of pH is less than 7.

(C)

• Calcium hydroxide ($\mathrm{Ca}{\left(\mathrm{OH}\right)}_2$) is a base of alkaline earth metal.
• Since the bases of alkaline earth metal are comparatively weaker than the bases of alkali metal ($\mathrm{NaOH}$), $\mathrm{Ca}{\left(\mathrm{OH}\right)}_2$ is a weaker base than $\mathrm{NaOH}$.
• Hence, the pH value of $\mathrm{Ca}{\left(\mathrm{OH}\right)}_2$ is less than $\mathrm{NaOH}$.

(D)

• Acetic acid (${\mathrm{CH}}_3\mathrm{COOH}$) is a weak acid as it partially ionizes in an aqueous solution to produce Hydrogen (${\mathrm{H}}^{+}$) ions.
• ${\mathrm{CH}}_3\mathrm{COOH}\left(\mathrm{aq}\right)\rightleftharpoons {\mathrm{CH}}_3{\mathrm{COO}}^{-}\left(\mathrm{aq}\right)+{\mathrm{H}}^{+}\left(\mathrm{aq}\right)$
• Since Acetic acid is weaker than $\mathrm{HCl}$, the pH value of ${\mathrm{CH}}_3\mathrm{COOH}$ is more than $\mathrm{HCl}$ but less than $\mathrm{Ca}{\left(\mathrm{OH}\right)}_2$.

Final Answer: Hence, option (B) is correct. $\mathrm{NaOH}$ has the highest value of pH.