In a solution of a weak acid AcOH, adding a salt containing the conjugate base (for example, sodium acetate) will cause which of the following?

In a solution of a weak acid AcOH, adding a salt containing the conjugate base (for example, sodium acetate) will cause which of the following?

• A. Increased dissociation of AcOH
• B. Suppression of dissociation of the weak acid and a decrease in [H+]
• C. No change in equilibrium
• D. The acid to completely ionize

Answer: (B)

The main concept is the common-ion effect for a weak acid. Acetic acid dissociates as HA ⇌ H+ + A−. When you add a salt that provides the conjugate base A−, you increase [A−] in the solution. Le Chatelier’s principle says the system will shift to counter this stress, moving the equilibrium toward the left to form more HA and reduce the amount of H+. So, the dissociation is suppressed and [H+] decreases (pH rises). Since acetic acid is weak, it does not fully ionize, and adding its conjugate base won’t cause complete ionization. The other options don’t fit because increasing dissociation isn’t happening, the equilibrium does change, and complete ionization isn’t achieved by this addition.

The main concept is the common-ion effect for a weak acid. Acetic acid dissociates as HA ⇌ H+ + A−. When you add a salt that provides the conjugate base A−, you increase [A−] in the solution. Le Chatelier’s principle says the system will shift to counter this stress, moving the equilibrium toward the left to form more HA and reduce the amount of H+. So, the dissociation is suppressed and [H+] decreases (pH rises). Since acetic acid is weak, it does not fully ionize, and adding its conjugate base won’t cause complete ionization. The other options don’t fit because increasing dissociation isn’t happening, the equilibrium does change, and complete ionization isn’t achieved by this addition.