The substance benzoic acid (C6H5COOH) is a weak acid (Ka = 6.3×10-5). What is the pH of a 0.232 M aqueous solution of sodium benzoate, NaC6H5COO? pH = This solution is (acidic, basic, or neutral.) b.) What is the pH of a 5.42×10-2 M aqueous solution of potassium acetate, KCH3COO? pH = This solution is (acidic, basic, or neutral.) c.) What is the pH of a 7.90×10-2 M aqueous solution of ammonium chloride, NH4Cl? pH = This solution is _ (acidic, basic, or neutral.
The Correct Answer and Explanation is:
a.) The pH of a 0.232 M aqueous solution of sodium benzoate, NaC₆H₅COO, is:
pH = 8.78
This solution is basic.
b.) The pH of a 5.42×10⁻² M aqueous solution of potassium acetate, KCH₃COO, is:
pH = 8.74
This solution is basic.
(Note: This calculation uses the standard Kₐ for acetic acid, 1.8×10⁻⁵.)
c.) The pH of a 7.90×10⁻² M aqueous solution of ammonium chloride, NH₄Cl, is:
pH = 5.18
This solution is acidic.
(Note: This calculation uses the standard Kₑ for ammonia, 1.8×10⁻⁵.)
Explanation
The pH of a salt solution is determined by the nature of the ions it forms upon dissolution in water. This process is known as salt hydrolysis. Salts formed from a strong acid and a strong base are neutral (pH = 7). However, when a salt contains an ion that is the conjugate acid of a weak base or the conjugate base of a weak acid, that ion will react with water to produce either hydronium ions (H₃O⁺) or hydroxide ions (OH⁻), thus altering the pH.
In parts (a) and (b), sodium benzoate and potassium acetate are salts of a strong base (NaOH/KOH) and a weak acid (benzoic acid/acetic acid). The cation (Na⁺/K⁺) is a spectator ion and does not affect the pH. The anion (C₆H₅COO⁻/CH₃COO⁻), being the conjugate base of a weak acid, will accept a proton from water in an equilibrium reaction:
A⁻(aq) + H₂O(l) ⇌ HA(aq) + OH⁻(aq)
This production of hydroxide ions (OH⁻) makes the solution basic (pH > 7). To find the pH, one must first calculate the base dissociation constant (Kₑ) for the anion using the relationship Kₐ × Kₑ = Kₒ (where Kₒ = 1.0×10⁻¹⁴). An ICE (Initial, Change, Equilibrium) table is then used to determine the equilibrium concentration of OH⁻, from which pOH is calculated (pOH = -log[OH⁻]). Finally, the pH is found using the formula pH = 14.00 – pOH.
In part (c), ammonium chloride is a salt of a weak base (NH₃) and a strong acid (HCl). The chloride ion (Cl⁻) is a spectator ion. The ammonium ion (NH₄⁺), the conjugate acid of a weak base, will donate a proton to water:
BH⁺(aq) + H₂O(l) ⇌ B(aq) + H₃O⁺(aq)
This production of hydronium ions (H₃O⁺) makes the solution acidic (pH < 7). The acid dissociation constant (Kₐ) for the ammonium ion is calculated from the Kₑ of ammonia (Kₐ = Kₒ / Kₑ). The equilibrium concentration of H₃O⁺ is then found, and the pH is calculated directly (pH = -log[H₃O⁺]).
