Examkin

Question

Difficulty: EasySalt Hydrolysis and Solution Acidity/Alkalinity

An aqueous solution of sodium fluoride (NaFNaF) is basic to litmus. Which species undergoes hydrolysis to cause this alkalinity?

  1. Fluoride ion (F−F^-)Answer
  2. B
    Sodium ion (Na+Na^+)
  3. C
    Hydroxide ion (OH−OH^-)
  4. D
    Hydrogen ion (H+H^+)

Answer

The fluoride ion (F−F^-) undergoes anion hydrolysis in water to produce hydroxide ions (OH−OH^-), resulting in a basic solution.
Sodium fluoride (NaFNaF) dissociates completely in water to yield Na+Na^+ and F−F^-. Because HFHF is a weak acid, its conjugate base (F−F^-) reacts with water molecules (anion hydrolysis) via F−+H2O⇌HF+OH−F^- + H_2O \rightleftharpoons HF + OH^-, generating hydroxide ions which make the solution alkaline.

Step-by-Step Solution

1
Identify the parent acid and base of the salt.
NaFNaF is formed from a strong base (NaOHNaOH) and a weak acid (HFHF).
Salts of strong bases and weak acids produce basic solutions upon dissolving in water.
2
Determine which ion reacts with water (hydrolyzes).
The fluoride ion (F−F^-) reacts with water according to F−+H2O⇌HF+OH−F^- + H_2O \rightleftharpoons HF + OH^-.
The conjugate base of a weak acid is strong enough to accept a proton from water.
3
Relate the generated ion to the acidity/alkalinity of the solution.
The production of extra OH−OH^- ions increases the pH above 7, turning the solution basic.
An excess of hydroxide ions relative to hydronium ions causes basic/alkaline behavior.

Key Concept

Anion Hydrolysis of Weak Acid-Strong Base Salts
Rate this question

Topics

Salt Hydrolysis and Solution Acidity/Alkalinity

More questions from this topic