Question

The highest pH value is of

• A. 0.1 M $NaCl$
• B. 0.1 M ${NH}_{4}Cl$
• C. 0.1 M ${CH}_{3}COONa$
• D. 0.1 M ${CH}_3{COONH}_4$

Answer 1

The correct option is C

0.1 M ${\mathrm{CH}}_3\mathrm{COONa}$

The explanation for the correct option (C):

• Salt is a compound formed from an acid by the replacement of the Hydrogen ($\mathrm{H}$) in the acid by a metal.
• Example: Hydrochloric acid ($\mathrm{HCl}$) is an acid. When the $\mathrm{H}$ of $\mathrm{HCl}$ is replaced by a metal, say Sodium ($\mathrm{Na}$), then it results in the formation of a salt Sodium chloride ($\mathrm{NaCl}$).
• $\mathrm{NaCl}$ is a salt. The aqueous solutions of many salts are neutral (pH = 7) but some salts form acidic or basic solutions when dissolved in Water (${\mathrm{H}}_2\mathrm{O}$).
• The acidic and basic nature of some salt solutions can be determined by the hydrolysis of salts.
• The salts of strong acids and strong bases produce solutions that are neutral in nature.
• The salts of strong acids and weak bases produce solutions that are acidic in nature.
• The salts of weak acids and strong bases produce solutions that are basic in nature.
• The salts of weak acids and weak bases produce solutions that are neutral in nature.
• The hydrolysis of ${\mathrm{CH}}_3\mathrm{COONa}$ produces Sodium hydroxide ($\mathrm{NaOH}$) and Acetic acid (${\mathrm{CH}}_3\mathrm{COOH}$). The equation is:

${\mathrm{CH}}_3\mathrm{COONa}\left(\mathrm{aq}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\to \mathrm{NaOH}\left(\mathrm{aq}\right)+{\mathrm{CH}}_3\mathrm{COOH}\left(\mathrm{aq}\right)$

• In this reaction $\mathrm{NaOH}$ is a strong base because it completely ionizes in Water to release Hydroxide ions (${\mathrm{OH}}^{-}$) and Acetic acid is a weak acid because it partially ionizes in Water to release Hydrogen ions (${\mathrm{H}}^{+}$). Thus, ${\mathrm{CH}}_3\mathrm{COONa}$ is a salt of a weak acid and a strong base. Hence, 0.1 M ${\mathrm{CH}}_3\mathrm{COONa}$ is basic with a pH value greater than 7 which is highest.

The explanation for the incorrect options:

(A)

• The hydrolysis of $\mathrm{NaCl}$ produces Sodium hydroxide ($\mathrm{NaOH}$) and $\mathrm{HCl}$. The equation is:

$\mathrm{NaCl}\left(\mathrm{aq}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\to \mathrm{NaOH}\left(\mathrm{aq}\right)+\mathrm{HCl}\left(\mathrm{aq}\right)$

• In this reaction $\mathrm{NaOH}$ is a strong base because it completely ionizes in Water to release Hydroxide ions (${\mathrm{OH}}^{-}$) and $\mathrm{HCl}$ is a strong acid because it completely ionizes in Water to release Hydrogen ions (${\mathrm{H}}^{+}$). Thus, $\mathrm{NaCl}$ is a salt of strong acid and strong base. Hence, 0.1 M $\mathrm{NaCl}$ is neutral with a pH equal to 7.

(B)

• The hydrolysis of ${\mathrm{NH}}_4\mathrm{Cl}$ produces Ammonium hydroxide (${\mathrm{NH}}_4\mathrm{OH}$) and $\mathrm{HCl}$. The equation is:

${\mathrm{NH}}_4\mathrm{Cl}\left(\mathrm{aq}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\to {\mathrm{NH}}_4\mathrm{OH}\left(\mathrm{aq}\right)+\mathrm{HCl}\left(\mathrm{aq}\right)$

• In this reaction ${\mathrm{NH}}_4\mathrm{OH}$ is a weak base because it partially ionizes in Water to release Hydroxide ions (${\mathrm{OH}}^{-}$) and $\mathrm{HCl}$ is a strong acid because it completely ionizes in Water to release Hydrogen ions (${\mathrm{H}}^{+}$). Thus, ${\mathrm{NH}}_4\mathrm{Cl}$ is a salt of strong acid and weak base. Hence, 0.1 M ${\mathrm{NH}}_4\mathrm{Cl}$ is acidic with a pH of less than 7.

(D)

• The hydrolysis of ${\mathrm{CH}}_3{\mathrm{COONH}}_4$ produces Ammonium hydroxide (${\mathrm{NH}}_4\mathrm{OH}$) and ${\mathrm{CH}}_3\mathrm{COOH}$. The equation is:

${\mathrm{CH}}_3{\mathrm{COONH}}_4\left(\mathrm{aq}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\to {\mathrm{NH}}_4\mathrm{OH}\left(\mathrm{aq}\right)+{\mathrm{CH}}_3\mathrm{COOH}\left(\mathrm{aq}\right)$

• In this reaction ${\mathrm{NH}}_4\mathrm{OH}$ is a weak base because it partially ionizes in Water to release Hydroxide ions (${\mathrm{OH}}^{-}$) and Acetic acid is a weak acid because it partially ionizes in Water to release Hydrogen ions (${\mathrm{H}}^{+}$). Thus, ${\mathrm{CH}}_3{\mathrm{COONH}}_4$ is a salt of a weak acid and a weak base. Hence, 0.1 M ${\mathrm{CH}}_3{\mathrm{COONH}}_4$ is neutral with a pH equal to 7.

Final Answer: Hence, option (C) is correct. The highest pH value is of 0.1 M ${\mathrm{CH}}_3\mathrm{COONa}$.