A salt is an ionic compound (cation and anion). Salts are very soluble in water. When they dissolve, the ions sometimes react with water and affect the pH of the solution; we call this hydrolosis. To determine how a salt affects pH, you must look at each of its ions individually:
- neutral (does not affect pH)
- Cation of a strong base (e.g., Na+ from NaOH) or anion of a strong acid (e.g., Cl− from HCl). Most of these ions are from group 1, 2, or 7, but don’t forget polyatomic ions like SO42− from H2SO4.
- acidic (decreases pH)
- Conjugate acid of a weak base (e.g., NH4+, the conjugate acid of NH3) or metal cation with high charge density (Al3+ and Fe3+ are the main ones).
- basic (increases pH)
- Conjugate base of a weak acid (e.g., CH3COO−, the conjugate base of CH3COOH).
When a salt contains both a cation that decreases pH and an anion that increases pH, we must compare their ionization constants to see which one overwhelms the other:
When a salt contains an amphoteric ion (it can donate a proton or accept one, e.g., HSO4−), you must write the equation twice: once for when the ion acts as an acid and once for when it acts as a base. Then, compare the and values the same way as shown above.
There is one more special case: oxides. A metal oxide always reacts with water to form a basic solution because one of the products is the hydroxide ion. Don’t write a dissociation equation—just react the compound with water directly. For example:
CaO(s) + H2O(l) → Ca2+(aq) + 2 OH−(aq).
A nonmetal oxide always reacts with water to form an acidic solution because one of the products is the hydronium ion. For example:
CO2(g) + H2O(l) → H2CO3(aq)
H2CO3(aq) + H2O(l) → HCO3−(aq) + H3O+(aq).
Will (NH4)2SO4(s) produce a neutral, acidic, or basic solution when dissolved in water?
First, we write the dissociation reaction, indicating that the salt dissolves completely with a one-directional arrow:
(NH4)2SO4(s) → 2 NH4+(aq) + SO42−(aq).
Since NH4+ is the conjugate acid of a weak base, we know that it will cause the solution to become acidic:
NH4+(aq) + H2O(l) ⇌ NH3(aq) + H3O+(aq), .
However, SO42−(aq) will also affect the pH: it is the conjugate base of a weak acid, so it will cause the solution to become basic:
SO42−(aq) + H2O(l) ⇌ HSO4−(aq) + OH−(aq).
We can’t just look up the ionization constant like we did for NH4+(aq) because it isn’t on the list, at least not directly. What is on the list is for H2SO4(aq), which is the same as for HSO4(aq). We can use this to find the ionization constant for the conjugate base of HSO4−(aq), which is SO42−(aq). Since ,
Now we have our and values for NH4+(aq) and SO42−(aq). They are 5.8 × 10−10 and 1.0 × 10−12 respectively. Since , dissolving this salt in water will result in an acidic solution.