Conjugates of Weak Acids & Salts

[Stephanie Cheung 2J]

When we write an equilibrium equation to figure out equilibrium concentrations, pH, etc, and we're not given the conjugate, there are times when we have to add a spectator ion to the conjugate. For example, for CH3COOH, the conjugate would be CH3COO-. But one of the UAs for the UA sessions I went to said that you can't have acetate on its own because it's reactive, so we add Na to make it NaCH3COO to stabilize it. How do we know when we are supposed to add Na or a spectator ion? I'm also kind of confused for how Na stabilizes CH3COO- since Na+ is a spectator ion.

[Brian Phung 2B]

The sodium salt of acetic acid (NaCH3COO) stabilizes CH3COO- when not in solution because the ionic bond between Na+ and CH3COO- cancels out the charge that makes it unstable. However, it does become a psetator ion when the salt is dissolved in water.
To conclude, you can ignore the spectator ions like Na+ or K+ when having to calculate p H because they are neutral in solution. Just treat NaCH3COO as CH3COO- in the reaction.