Question #823c6

1 Answer
Jun 4, 2015

The pH of your solution will be equal to 8.90.

Sodium fluoride, NaF, is a soluble salt that dissociates completely in aqueous solution to give sodium cations, Na+, and fluoride anions, F.

NaF(s)Na+(aq)+F(aq)

Since 1 mole of sodium fluoride produces 1 mole of fluoride anions, the concentration of the fluoride ions will be equal to that of the salt.

[F]=[NaF]=0.22 M

The fluoride ions will react with water to form hydrofluoric acid, a weak acid, and hydroxide ions, OH, which is an indicator that the pH of the solution will be greater than 7.

Use an ICE table to determine the concentration of the hydroxide ions

F(aq)+H2O(l)HF(aq)+OH(aq)
I....0.22...............................0..................0
C....(-x)...............................(+x)..............(+x)
E...0.22-x............................x...................x

The base dissociation constant, Kb, will be equal to

Kb=KwKa=10143.5105=2.861010

This means that you'll get

Kb=[HF][OH][F]=xx0.22x=x20.22x

Since Kb is so small, you can approximate (0.22-x) with 0.22 to get

Kb=x20.22=2.861010

Therefore,

x=0.222.861010=7.9106

You can now determine the pOH of the solution

pOH=log([OH])

pOH=log(7.9106)=5.10

As a result, the solution's pH will be

pHsol=14pOH=145.10=8.90