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:
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 `K_"a"` and `K_"b"` 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), `K_"a" = 5.8xx10^-10`.
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 `K_"a2"` for H2SO4(aq), which is the same as `K_"a"` for HSO4(aq). We can use this to find the ionization constant for the conjugate base of HSO4−(aq), which is SO42−(aq). Since `K_"a"K_"b" = K_"w"`,
`K_"b" = K_"w"/K_"a2" = (1.0xx10^-14)/(1.0xx10^-2) = 1.0xx10^-12`.
Now we have our `K_"a"` and `K_"b"` values for NH4+(aq) and SO42−(aq). They are 5.8 × 10−10 and 1.0 × 10−12 respectively. Since `K_"a" > K_"b"`, dissolving this salt in water will result in an acidic solution.