What is the pH of 0.10 M NaF (aq)? Show full solutions

Answer 1

NaF is the salt of a strong base (NaOH) and a weak acid (HF). Therefore this salt will have a basic (>7) pH.

To find the pH of this solution, we look at the hydrolysis of the salt...

NaF +H₂O ==> NaOH + HF ... full molecular equation

F^- + H₂O ==> HF + OH^- ... net ionic equation

Here, F^- is acting as a base, and so we will use the Kb expression and solve for [OH^-]

To get the Kb we can use the Ka for HF which is 7.2x10^-4 (I looked it up)

KaKb = 1x10^-14

Kb = 1x10^-14 / 7.2x10^-4

Kb = 1.39x10^-11

Kb = [HF][OH^-] / [F^-]

1.39x10^-11 = (x)(x) / 0.10

x² = 1.39x10^-10

x = 1.18x10^-5 M = [OH^-]

pOH = -log 1.18x10^-5 = 4.93

pH = 14 - pOH = 14 - 4.93

pH = 9.07 (alkaline, as predicted)