What is the pH of a 2.16 M LiF solution?

Question

What is the pH of a 2.16 M LiF solution?

Answer 1

LiF is made from a strong base (LiOH) and a weak acid (HF). Therefore, the pH will be basic (alkaline).

To determine the pH, we must look at the hydrolysis of F^-. We also need the Ka for HF, which from my source is 7.2x10^-4.

F^- + H2O ==> HF + OH^- (F^- is acting as a base so we need the Kb)

KaKb = 1x10^-14 and Kb = 1x10^-14/Ka = 1x10^-14 / 7.2x10^-4 = 1.39x10^-11

Kb = [HF][OH^-] / [F^-] = (x)(x) / 2.16 - x and we can ignore x in denominator as it is small relative to 2.16

1.39x10^-11 = x² / 2.16

x² = 3.00x10^-11

x = 5.48x10^-6 = [OH^-]

pOH = -log 5.48x10^-6 = 5.26

pH = 14 - 5.26

pH = 8.74