Ammonium chloride is a salt that forms when ammonia (NH3) reacts with hydrochloric acid (HCl). The pH of a solution of ammonium chloride depends on the concentration of the salt. At room temperature, a 0.1 M solution of ammonium chloride has a pH of around 5.0, which is slightly acidic. This is because the ammonium ion (NH4+) undergoes hydrolysis in water to form hydroxide ions (OH-), which increases the pH of the solution. The reaction that takes place is:

NH4+ + H2O ⇌ NH3 + H3O+

The equilibrium constant for this reaction is Ka = [NH3][H3O+] / [NH4+], and the value of Ka is 5.6 * 10-10 at 25°C. This means that the concentration of hydroxide ions in a 0.1 M solution of ammonium chloride is:

[OH-] = Kw / [H3O+] = 1.0 * 10-14 / (1.0 * 10-5) = 1.0 * 10-9 M

The pH of the solution can be calculated from the concentration of hydroxide ions using the equation:

pH = -log[H3O+] = -log(Kw / [OH-]) = -log(1.0 * 10-14 / 1.0 * 10-9) = 5.0

Therefore, the pH value of a 0.1 M solution of ammonium chloride is around 5.0.