Kb question (1 Viewer)

[pixiefeathers]

Hi, why is the Kb of sodium acetate the same as the Kb of the acetate ion itself? I know that sodium acetate is basic because it has the conjugate base of a weak acid (acetic acid) and sodium is neutral, but I also thought that sodium acetate would be a different base compared to just the acetate ion so its Kb would be different. I have the same question for HCOO- and HCOONa. Does anyone know what is wrong with my understanding? Thank you.

 

[wizzkids]

You have answered your own question! Water is pH7 because it contains 10^-7 mol/L H⁺ and 10^-7 mol/L OH^- ions. The sodium ion in aqueous solution has negligible effect on the H⁺/OH^- equilibrium in water. Only the acetate ion will have an effect on the equilibrium. The same reasoning applies to the pH of sodium methanoate.
Does that help, or is that just confusing you?

 

[pixiefeathers]

Oh, does the sodium ion dissociate from the acetate ion immediately in water and doesn't change the base? I just thought that sodium acetate, as a base, will engage in ionisation/acid-base reactions differently to just the acetate ion? Or is it because the sodium ion doesn't participate in H+ transfer so it's just a spectator ion that does nothing?

 

[wizzkids]

Yes. The sodium ion is just a spectator. Potassium is also a spectator ion. The hydrated metallic cations are not all spectators. Some can have a dramatic effect on pH. The iron III (ferric) ion will lower the pH a lot. Nickel II and manganese II ions will raise the pH.