At 25 °C, what is the pH value of a salt made up of a strong acid and a weak…

Concept

A salt is the ionic product formed when an acid and a base neutralise each other, and the pH of its aqueous solution is decided by salt hydrolysis. An ion derived from a weak parent can react with water and produce H+ or OH−; an ion derived from a strong parent is the conjugate of a fully ionised species, is itself extremely weak, and hydrolyses negligibly. For a salt formed from one strong and one weak parent, the solution's character is therefore dictated by the ion from the weak parent, because it is the only ion that appreciably disturbs the water equilibrium.

Applying it to this salt

1. The salt in question comes from a strong acid and a weak base. The standard textbook example is ammonium chloride, NH4Cl, formed from HCl (a strong acid) and NH3 (a weak base).

2. In water it dissociates completely into its two ions: NH4Cl gives NH4+ plus Cl−.

3. The anion Cl− is the conjugate base of the strong acid HCl. Because HCl gives up its proton completely, Cl− has essentially no tendency to take a proton back from water, so it does not hydrolyse and contributes nothing to the pH.

4. The cation NH4+ is the conjugate acid of the weak base NH3. Because NH3 is a weak base, so its conjugate acid NH4+ transfers a proton to water: NH4+ + H2O gives NH3 + H3O+.

5. That hydrolysis creates a net hydronium excess without any net hydroxide production by the salt, so the concentration of H3O+ exceeds the concentration of OH−. The solution is therefore acidic, which on the 25 degree Celsius scale means a pH below 7.

6. Quantitatively, for such a salt pH = half of (pKw minus pKb minus log C). For a 0.1 M solution of NH4Cl, with pKb of ammonia equal to 4.75, this works out to a pH of about 5.1: clearly on the acidic side of neutral, and nowhere near the strongly alkaline 10 to 14 band.

Cross-check against the other salt families

The same hydrolysis rule predicts every case, which is the fastest way to confirm the result:

Result: a salt made from a strong acid and a weak base gives an acidic solution, so its pH value is less than 7.