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Q.
Which of the following will supress the ionisation of acetic acid in aqueous solution ?
Equilibrium
Solution:
Common ion effect is observed when a solution of weak electrolyte is mixed with a solution of a strong electrolyte, which provides an ion common to that provided by the weak electrolyte.
Acetic acid is a weak electrolyte. $HCl$ is a strong electrolyte. $HCl$ provides hydrogen ion which is common to that provided by acetic acid.