# Common ion effect

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

## 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

Gold Member
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.