http://www.ck12.org Chapter 21. Acids and Bases
F−(aq)+H 2 O(l)⇀↽HF(aq)+OH−(aq)The fluoride ion is acting as a weak Brønsted-Lowry base. The hydroxide ion that is produced as a result of the
above reaction makes the solution slightly basic.Salt hydrolysisis a reaction in which one of the ions from a salt
reacts with water, forming either an acidic or basic solution.
Salts That Form Basic Solutions
When solid sodium fluoride is dissolved into water, it completely dissociates into sodium ions and fluoride ions. The
sodium ions do not react with water, but a minor portion of the fluoride ions will deprotonate water, producing small
amounts of hydrofluoric acid and hydroxide ions.
F−(aq)+H 2 O(l)⇀↽HF(aq)+OH−(aq)The pH of the resulting solution can be determined if the Kbof the fluoride ion is known.
Sample Problem 21.7: Salt Hydrolysis
20.0 g of sodium fluoride is dissolved in enough water to make 500.0 mL of solution. Calculate the pH of the
solution. The Kbof the fluoride ion is 1.4× 10 −^11.
Step 1: List the known values and plan the problem.
Known
- mass of NaF = 20.0 g
- molar mass of NaF = 41.99 g/mol
- volume of solution = 0.5000 L
- Kbof F−= 1.4× 10 −^11
Unknown
- pH of solution =?
The molarity of the F−solution can be calculated from the mass, molar mass, and solution volume. Since NaF
completely dissociates, the molarity of the NaF is equal to the molarity of the F−ion. An ICE table can be used to
calculate the concentration of OH−produced, and the pH of the solution can be determined from that value.
Step 2: Solve.
- (^0) g NaF×
(^1) mol NaF - (^99) g NaF
×
1 mol F−(^1) mol NaF
= 0 .476 mol F−
0 .476 mol F−
0 .5000 L
= 0 .953 M F−
Hydrolysis equation: F−(aq)+H 2 O(l)HF(aq)+OH−(aq)TABLE21.10:ICE Table
Concentrations [F−] [HF] [OH−]
Initial 0.953 0 0
Change −x +x +x
Equilibrium 0.953−x x x