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Homework Help: Common ion effect

  1. Mar 6, 2008 #1
    1. The problem statement, all variables and given/known data

    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
    2. Relevant equations

    3. 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
  2. jcsd
  3. Aug 22, 2017 #2


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    Science Advisor
    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.
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