The solution of which one of the following will have pH less than 7?

The pH of a solution depends on the nature of the salt and its hydrolysis. Salts formed from a strong acid and a strong base, such as Sodium chloride (NaCl) and Potassium chloride (KCl), are neutral with a pH of approximately 7 [1]. Sodium hydroxide (NaOH) is a strong base, which significantly raises the pH above 7 [1]. In contrast, Ferric chloride (FeCl3) is a salt of a strong acid (HCl) and a weak base (Fe(OH)3). When dissolved in water, the ferric ions (Fe3+) undergo hydrolysis, reacting with water to produce hydrogen ions (H+) and iron(III) hydroxide. This process increases the concentration of hydrogen cations, resulting in an acidic solution with a pH less than 7. Therefore, FeCl3 is the only substance among the options that produces an acidic aqueous solution.

Sources

1. [1] Science , class X (NCERT 2025 ed.) > Chapter 2: Acids, Bases and Salts > Activity 2.14 > p. 29