4-Nitrophenol, NO2C6H4OH (pKa 7.15), is only slightly soluble in water, but its sodium salt, NO2C6H4O-Na+, is quite soluble in water. Describe the solubility of 4-nitrophenol in solutions of sodium hydroxide, sodium bicarbonate (NaHCO3), and sodium carbonate (Na2CO3). The pKa values for the conjugate acids of sodium hydroxide, sodium bicarbonate (NaHCO3), and sodium carbonate (Na2CO3) are 15.7, 6.36, and 10.33, respectively. Aqueous NaOH: Aqueous NaHCO3: Aqueous Na2CO3:

Aqueous NaOH:     soluble

Aqueous NaHCO₃: insoluble

Aqueous Na₂CO₃:  soluble

Explanation:

The organic acid is insoluble. Its salt (ionic) is soluble.

The important principle is:

If you have two acids in a flask, the stronger acid (smaller pKₐ) will protonate the weaker one. The stronger acid will become ionic and therefore more soluble.

1. In NaOH

Let's write the formula for 4-nitrobenzoic acid as HA.

The equation for the reaction is

        HA  +  OH⁻ ⇌ A⁻ + H₂O

pKₐ:  7.15     15.7

HA is the stronger acid. It will protonate the hydroxide ion and be converted to the soluble 4-nitrobenzoate ion.

4-Nitrophenol is soluble in NaOH.

2. In NaHCO₃

        HA  +  HCO₃⁻ ⇌ A⁻ + H₂CO₃

pKₐ:  7.15     6.36

HCO₃⁻ is the stronger acid. It will protonate 4-nitrophenol.

4-Nitrobenzoic acid is insoluble in NaHCO₃.

3. In Na₂CO₃

        HA  +  CO₃²⁻ ⇌ A⁻ + H₂CO₃

pKₐ:  7.15    10.33

HA is the stronger acid. It will protonate the carbonate ion.

4-Nitrophenol is soluble in Na₂CO₃.