Common Ion Effect: pH Change for LiF, KI & NH4Cl

[tribalman100]

Homework Statement

does the ph increase , decrease or remain the same on addition of each of the folling
a) LiF to an HF Solution
B) KI to an HI solution
C) NH4Cl to an NH3 solution

Homework Equations

The Attempt at a Solution

i got the equations and the answers to these i just want to know the reason so i can do it next time on why Lif ph increases, Ki's remains the same and that NH4Cl's decreases . it would be a great help and this explanation might save my chemistry grade. thanks for the help guys

 

[TeethWhitener]

These are all applications of Le Chatelier's principle.
a) HF is a weak acid, so in water, the dissociation is incomplete:
$$HF \rightleftharpoons H^+ + F^-$$
Adding LiF to the solution increases the concentration of F^- ions, which shifts the HF equilibrium to the left. This indicates a decrease in [H⁺], and therefore an increase in pH.

b) HI is a strong acid in water, so it dissociates completely, meaning that adding the common I^- ion via KI doesn't affect the pH.

c) NH₃ is a weak base. It reacts with water in an equilibrium:
$$NH_3 + H_2O \rightleftharpoons NH_4^+ + OH^-$$
Adding NH₄Cl pushes the equilibrium to the left, decreasing OH^- concentration and consequently decreasing pH.